First, find and check in with the production coordinator, Castifi on-set coordinator, or 2nd AD. Your timecard will start as soon as the production coordinator checks you in. There's no need to start or end your timecard in the Castifi app - the production will do this on their end.
After you arrive on set and check in, a member of the production team will direct you to the area where you will be waiting, also known as "holding." You may need to fill out paperwork for the production if it was not sent to you digitally by Castifi. Be sure to carefully, thoroughly, and neatly fill out any paperwork you're given by the production. Turn in your paperwork to the production coordinator, Castifi on-set coordinator, or 2nd AD when you're done.
The production may direct you to hair and makeup before you're used in a scene. You may also work with the production's wardrobe team to find the right look for a scene. Make sure you're prepared with the right hair, makeup, and wardrobe options - these are all listed in the Castifi app.