Upper Body
Forearms
Place the lacrosse ball between your forearm and an elevated surface (wooden box shown here). Using an elevated surface allows you to effectively position yourself to apply pressure as required.
Lean into the ball as much as is reasonable comfortable.
You can stay in place and maintain, - or increase - downward force on the lacrosse ball; or, you can also introduce rolling back and forth.
Upper Body
Traps
While laying on your back (knees bent or straight), place the lacrosse ball on the middle or lower portion of your upper trapezius muscle.
You can stay in place and maintain - or increase, downward forward force on the lacrosse ball.
You can also introduce rolling back and forth.
Another option: move your arms from being by your side to the overhead position (palm facing inward and thumb touching the ground).
Lower Body
The Calf
Place the lacrosse ball between your calf and the floor.
Lean into the ball as much as is reasonable comfortable.
You can choose to stay in place and maintain, or increase, downward forward force on the lacrosse ball.
You can also introduce rolling back and forth as well pointing and flexing your foot.