What is the proper way to use a convertible infant seat as a belt positioning booster?

So my child has exceeded the size to use the straps on his car seat and it is time to use it with a seat belt instead. The instructions on this aren’t entirely clear. It seems to me, since it can be used in a car without LATCH, that it must be appropriate to set the seat in the car seat, put the child in it, and seat belt the whole thing in place. However, given that my car does have LATCH, should I strap the seat in with LATCH first?