First step is to list the available db's to figure out which one to describe and then copy the connectionName for.
gcloud sql instances list
Assuming the db you want is called foo you would then copy the connectionName as follows.
gcloud sql instances describe foo | grep connectionName | awk '{print $2}' | xclip -selection clipboard
Download cloud-sql-proxy here.
Optionally move cloud-sql-proxy to your bin such that it's path accessible with the following command.
sudo mv cloud-sql-proxy /usr/local/bin
Setup application default credentials using this gcloud guide linked here.
Finally use the cloud-sql-proxy to forward a local port such that the db can be accessed locally.
cloud-sql-proxy --port 6432 {replace with the copied connectionName}
For something a bit more automatic. This uses
fzfto select the instance name. Useful if you're a maniac and have multiple SQL instances.