assuming you are already installed flutter and jdk 8.

first download command line tools

download the Command line tools only.
link:
https://developer.android.com/studio/index.html#downloads

cd into your home directory and create folder android_sdk/cmdline-tools.
extract the downloaded command line tools into the newly created folder cmdline-tools.
then rename the extracted folder to latest.

now it should be something like this:

$ ls $HOME/android_sdk/cmdline-tools/latest
bin  lib  NOTICE.txt  source.properties

add this to .bashrc file, or whatever shell you are using

export ANDROID_SDK_ROOT=$HOME/android_sdk
export PATH=$PATH:$ANDROID_SDK_ROOT/cmdline-tools/latest/bin
# prepared
export PATH=$PATH:$ANDROID_SDK_ROOT/platform-tools
export PATH=$PATH:$ANDROID_SDK_ROOT/tools
export PATH=$PATH:$ANDROID_SDK_ROOT/emulator

then, restart your terminal if it is running. or start a new terminal if not.

run the following command:

sdkmanager "platforms;android-30"
sdkmanager "patcher;v4"
sdkmanager "tools"
sdkmanager "build-tools;29.0.2"
sdkmanager --update

it will download some big files, just wait until it complete.
then run this command:

flutter config --android-sdk $ANDROID_SDK_ROOT
flutter doctor --android-licenses

it will ask something (y/n), just y all.

now try running your flutter app. good luck

Addition

if you are using scrcpy, and it doesn't work properly when running flutter,
make sure the adb version you are using is the same as the one in android sdk.

to fix it, copy the adb binary located in $ANDROID_SDK_ROOT/platform-tools (used by flutter) to /usr/bin (used by scrcpy).
for example:

sudo cp $ANDROID_SDK_ROOT/platform-tools/adb /usr/bin

alimsk/README.md

Addition

References