This guide serves as a beginner-friendly introduction to update skipping and how to use it.
This guide assumes you have a basic understanding of redstone and block updates.
Block update - an interaction that generally informs the game something physically changed. Block update detector - something that detects a block update. Quasi connectivity - the functionality of some redstone components to power like doors.
Update skipping is a mechanic that allows block updates to be skipped when the block update queue, configurable in the server settings, goes over a specific point, and then is unable to add further updates to the queue, hence skipping them. For example, you could break an obsidian block next to a portal, that updates the skipper, and then one million updates are executed in the skipper, meaning only the neighboring blocks near the point that was skipped would update, and the rest would be skipped.
The common uses for it are floating snow (e.g. to block light) and portal slicing, to create illegal portal configurations, usually for mob farms. A few other usages are floating signs, rails, half-doors etc, but they don't have much real usage compared to floating snow or portal slicing.
There are a few reliable update skipper designs, and most of them either use dust updates, or wall updates.
Redstone dust based skippers use large amounts of redstone dust configured to change state on a block update using detector rails, as they don't update the redstone dust when retracted from under minecart by pistons, hence creating dust that depowers instantly when updated in locations next to where the detector rail was retracted. Redstone dust updates an absolutely massive amount of times, including redundant updates (read more at https://github.com/SpaceWalkerRS/alternate-current/blob/main/README.md), hence allowing generation of a queue of a million updates from only 16-18 thousand redstone dust, allowing very dense skippers that are clean to build.
While redstone dust bases skippers require considerably more materials, the build effort is significantly reduced, and they do not need bedrock roof breaking in the nether or weird rail placement for instant wires.
For example, here's Infernal's dust-based skipper: 
Wall skippers rely on mechanics added in 1.16 that allows walls to change the shape of other walls under them, when the shape of the topmost wall is changed. This can be used to chain large amounts of updates, by updating a massive column of walls at once. Wall skippers save a lot of walls by updating the walls multiple times within the same chain, this is how they work: The block update required to skip updates an instant wire, that updates trapdoors, that change the shape of the topmost wall updating all walls under it, and then instantly resets the trapdoor, to reset the wall for the next module. The next module then detects the update while being in the same chain, and updates the walls yet again, continuing the chain for next module, and so on, until 1 million updates is hit. Wall skippers are a lot less dense than redstone dust based skippers, but require 3-4x less blocks to place, however their very tall size makes them annoying to build and to transfer updates up from the bottom to.
These skippers take a lot less blocks to build but need annoying rail placement, and would need bedrock roof breaking in the nether, so they are primarily better for overworld skipping.
For example, here's HaoHandy's wall based skipper: 
WARNING THIS DESIGN ONLY WORKS 1.16-1.20-prerelease1
This is a very small skipper design (and suppressor pre-1.19, but I will not go into detail on it) It works by recursively making dust redirect to the dust on top of trapdoor, powering it, redirecting away from trapdoor due to it being open, then trapdoor closing and redirecting to it again, creating an insane amount of updates until the game suppresses updates due to too many in pre 1.19, or hits the 1 million update limit in 1.19+ and skips updates. However, mojang has made dust break when trapdoors open under it in 1.20-pre2 hence this skipper no longer works.
Picture of an example setup using this skipper: 
There are some solid designs, however a skipper is not the only component you need to update skip.
Most skippers can only skip in a very short rail line by default, however there are now many methods to "extend" the distance where they can skip updates, usually known as arms/budlines.
There are 3 types of (useful) budlines: Vertical, Main Arm, Movable Arm
This is (usually) a tower of noteblocks that are BUD-ed with dust-redirection (redirecting dust with target blocks does not update the blocks nearby, so it allows components to be in the wrong state of power) that can be updated from any noteblock in the tower, and transfers that update to the skipper, letting you update skip from Y-levels below or above the skipper.
For example, this is a solid wallstone based vertical budline: 
This is usually a line of rails that are BUD-ed and are connected to the skipper through a vertical budline or directly, and it allows skipping updates, but it cannot be moved other than rebuilding it. It can be updated from any point in the line, allowing for a connection to a movable "arm".
For example, this is a common main arm design: 
This is also a BUD-ed line of rails, however the difference is that it can be moved, allowing you to skip in many areas easily instead of having to constantly rebuild an arm. It is connected to the main arm, and can be updated from any point along the line of rails, allowing easy update skipping.
For example, a solid movable arm is PFLA's movable cce suppressor arm: 
Usually, movable budlines are simply rotated 90° from the main arm and the rails are directly touching each other, allowing the rail to update the main arm.
A main arm is usually connected to a vertical budline by having the rail line touch the noteblocks, letting it transfer the updates further
The vertical budline is usually connected to the skipper by being connected to it through a short budline that actually updates the dust/instant wires for trapdoors.
Haohandy 6gt dust skipper with arm and vertical budline included: https://discord.com/channels/949984617300123649/1006269415572123709/1272230559992774768
Infernal 10gt dust skipper with arm and vertical budline included: https://discord.com/channels/949984617300123649/1006269415572123709/1271562339904262235
Personally, I like PFLA's skipper arm and HaoHandy 6gt dust skipper, which is one of the complete setups linked above
Most of these skippers are directional, so please do not rotate them. Infernal's 10gt reset skipper also requires chunk alignment, shown by the post I linked. Infernal's skipper is not directional, but the arm is, hence I really don't recommend rotating or mirroring any of these skippers or setups. The Dust-Trapdoor skipper has to be built with the comparator in north-south direction.
https://youtu.be/vZhkyMnjrTA - PFLA's introduction to bud-lines and arms
https://youtu.be/sHDIe9rDQME - PFLA's movable CCE suppressor arm (also works for skippers)
https://youtu.be/81S7Q-O28X8 - Igna's video on the concept
https://youtu.be/PJWk5_Ng1AA and https://youtu.be/DKkuJ4dr3lQ - Infernal's slicing layout guides for slime chunk portal slicing
https://youtu.be/zbJn_TFjGT4 - Infernal's layout for 1x1 portal
https://youtu.be/ffNg5dixhXo - sussy?????
https://youtu.be/GuD-2spz_uY - Infernal budline and layout for slicing portal with 2 separation block
https://youtu.be/UT7IONA4qtE - Igna's tutorial on how to slice portals for the EOL mob farm
https://youtu.be/NfVczIqYT1U - Infernal's 2gt reset Dust-Trapdoor Skipper
https://discord.com/channels/949984617300123649/1006269415572123709/1260424125122613270 - Infernal's 10gt reset Dust Skipper (Saints of Suppression discord)
https://www.bilibili.com/video/BV1WhYxeWE3K/ - HaoHandy 6gt Dust Skipper
https://b23.tv/t2uil17 - HaoHandy 6gt Wall Skipper
https://youtu.be/aie36sEZv1M - enoki Love 12gt Wall Skipper
Igna778 and FX: Discovering update skipping (kinda)
PFLA: being cool
Shadowed: slight changes to wording
Infernal: cool skipper(s)
Based