I made this to go with Squid_ward's floating island instructable. View it here: https://www.instructables.com/id/Floating-Island/. Now, to the point all you will need is a floating island, a slime block, a sticky piston, and a lever.
Step 1: Make a Floating Island
Make a floating island. https://www.instructables.com/id/Floating-Island/
Step 2: Marking Your Spot
Jump about 1 block of your island.
Step 3: Forming the Bounce Pad.
Place a sticky piston directly below where you landed, place a slime block on top, then add a lever next to the slime block.
Step 4: Testing and Fixing
Test the bounce pad by standing on top of the slime block and flicking the lever. Then, edit the island so you can get on.