What I've found most useful when designing set-based vs row-based coding, is a few tips:

1. Assume rows don't have an order to them. So you can't do "row 1, then row 2, then row 3". Do rows 1-3 all at once.

2. Treat sets of rows as a single object, and call methods against them, instead of doing something procedural to individual rows. Methods include calculations, aggregations, CRUD, and so on. Think of the From clause in a query as defining the object, and the other clauses (Select, Update, Where, Order By, etc.) as methods against that set-of-rows object.

3. The concept of working against columns instead of rows also helps.

There's more to it, but that's a good start.