Once you have your bra, assess your garment and decide if you want to cut off the straps and cut out each cup individually, cut the sides off but leave the cups connected at the middle, or leave the middle and most of the sides and just cut off the closure on the back.
Try your clothing on and pin your bra/cups in place at the location where you want it to be.
If the top edge or side of your cup is sticking out to where you can see it, you can fold over that edge inward and tack it in place with a needle and thread (the folded edge shouldn’t really bother you since it’s soft foam).
Or, you can cut the edge and sew alongside it with a zigzag stitch or a serger if you have one to keep it from unraveling further.
Once your cup is in place, use a needle and thread to attach each cup to the garment in at least four places about 1/4″ away from the cup edge (at each side and top and bottom). You can do more locations on the cup if you feel like it’s needed.
If you left on some of the sides of the bra, you’ll want to tack those areas down as well.
When attaching the cup, use a thread color that will be least noticeable on the outside of the garment and either make very small stitches to where you are coming through to the front and going back in almost at the same spot.
Or, if your garment has two layers of fabric in that area, just catch the inside layer so no stitches are noticeable from the outside.
Once your bra is in, your item is ready to wear!