Stand your horse square if possible. Using strong finger and nail pressure on the midline of the belly right up near the elbow, begin wiggling or digging your fingers into the midline. Use your legs to lift your hands, not your arms. You're looking to see the wither area lift up. Move down the centerline towards the umbilicus and watch for the same lifting action, you can keep the pressure on or not.
For a bum tuck you scritch your fingers in about 4" down and at about a 45 degree angle from the tail head - probably at the point of the buttocks.
Don't stand right behind your horse!
There's probably other ways to do it.