You simply say, “St. (Name), pray for us” or “St. (Name), please pray for my intention” or “St. (Name), please help me with this.”
You can have a little conversation with them if you like, as you would with a friend. You can light a candle in front of their statue as part of your prayer or in thanksgiving for their help. It’s good to say Thank you to them sometimes.
You can find novenas to them online, sometimes chaplets, and other special prayers like Prayer to St. Michael.
You could write them letters or journal. I prefer to just talk to them though, since I lack time to write.