The app, step by step. The rules are on their own page; this is what you'll see and what to tap.
Someone deals a game and picks Rummy 500 at the top of New game (two to four of you). Say Deal me in; the table deals itself when everyone has answered.
The pile lies spread across the middle so every card in it shows; the stock sits beside it with its count. Sets and runs go above, each with the name of whoever put it down. The strip keeps everyone's total.
Tap the stock to draw one, or take from the pile. Plain — what a new table deals — takes the one card on top, and it's yours with nothing owed (you just can't discard it again this go). Deep pile takes any card in the pile and everything above it: a card lights up only when you could use it, and once you take it, it goes down first. Whichever your table chose, the app only lights up a take you can make.
Tap three or more cards that make a set or a run, then Put down. Tap one card and then a set or run on the table to add it there. When you're done, tap a card and Discard — that ends your go.
When someone goes out, or the stock runs dry, the feed says what everyone put down and what they were caught holding, and the next hand is dealt straight away.
When someone reaches 500 the results card comes up: highest first, the totals, and the hat on the lowest. Rematch deals the same table again.