There aren’t any specific positions that work best for anal stretching and training. What’s more important is what feels comfortable for you. Understanding your anatomy, with all its internal angles and curves, sets the stage for successful dilating and play.
You’ll want direct, straight-in access, paying attention to your sphincter muscles upon entry. This means that it may be best lying on your back or on all fours and using a repetitive motion, going in-and-out, until you hit resistance. This controlled pressure, with continued gaping, will allow for the relaxation we need that will assist in anal stretching. You may need a bit of trial and error to see what feels right. I also encourage you to switch things up because we want to simulate future play as best we can and sometimes that takes on a mind of its own–or many different positions.