Place 2 solid blocks side by side for the base
Build 2 vertical columns of blocks as the frame
Leave a 1-block gap between the columns for the elevator shaft
Make the shaft as tall as you want the elevator to go
Place a floor block at the bottom inside the shaft
Add a sticky piston facing upward under the floor block
Place a redstone block on top of the sticky piston
Add a lever or button on the side of the frame
Connect the lever or button to the piston with redstone dust or a redstone torch setup
Activate the lever or button to raise the floor block
Add another sticky piston and redstone block at the top if you want a return mechanism
Use slabs, trapdoors, or fences to keep the player centered
Test the elevator by standing on the floor block and activating the mechanism
Repeat the piston setup for multiple levels if needed