Place a vertical stack of blocks for the elevator shaft
Leave a 1-block-wide space for the elevator path
Place a sticky piston at the bottom facing upward
Place a slime block on top of the sticky piston
Place another slime block above it if you want more height
Add a solid block on the side for the redstone input
Place a button or lever next to the bottom block
Connect the button or lever to the sticky piston with redstone dust
Press the button or lever to push the slime block platform upward
Build a matching landing platform at the top
Add a second sticky piston and slime block setup at the top if you want a return elevator
Use honey blocks instead of slime blocks if you want no sticking to nearby blocks
Surround the shaft with walls to keep the platform aligned
Test the elevator and adjust the redstone timing if needed