Simply rightclick to launch yourself. Direction is similar to the "Slimesling" from Minecraft's Tinker's Construct mod (opposite your facing direction). Leftclicking will change the sling's power, and shift-leftclicking will display the sling's power HUD element. Pressing shift mid-flight will cancel your current trajectory, allowing you to sling again!
Make a "Y" pattern of dirt_with_grass nodes on a 3x3 crafting grid and place a bucket of water in the top-middle
Pummel a wet sponge and dirt together with your strongest mallet.