Writing a pack
A pack needs prompts, the cards with a blank in them, and answers, the cards that fill the blank. You need at least one prompt and thirty answers for a pack to be playable, which sounds like a lot until you start writing about people you know and cannot stop.
Playing it
Custom packs appear in the pack list when you set up a game, alongside everything else you own. Mix them with the standard packs or play a game entirely from your own cards.
Sharing
Anyone in a game with you can play your pack. They do not need to own it, buy it, or subscribe to anything. Group chat packs, work leaving-do packs, and wedding packs are all things people build here, mostly at the expense of one specific person.