For Kimai2
First clone it to your Kimai installation plugins directory:
cd /kimai/var/plugins/
git clone https://github.com/delftsolutions/kimai-vacation-hours.git VacationHoursBundle
And then rebuild the cache (As described on https://www.kimai.org/documentation/cache.html). cd into the top Kimai directory and execute the following commands:
bin/console kimai:reload --env=prod
chown -R :www-data .
chmod -R g+r .
chmod -R g+rw var/
You probably need to chown and chmod based on your local setup.