Slime Blocks can be acquired with 9 Slime Balls.
Slime Balls are obtained by killing the "Slime" mob. Slime are found in swamps during full moons, or in caves in certain chunks, although they are fairly uncommon because of how they spawn.
After you obtain 9 Slime Balls, go to a crafting table and locate the Slime Block, craft it, and find an area to pillar up. Place the Slime Block and build a pillar straight up next to the Slime Block using a whole 2 stacks of whatever block you want. Then... fall onto the Slime Block and your achievement should pop shortly after!
note: make sure you don't hold the jump button while falling onto the Slime Block, this will negate your fall damage.