Gather blocks, water buckets, kelp, soul sand, and a door or signs
Build a vertical shaft from the bottom to the top
Place a door or signs at the bottom opening to hold water in place
Fill the entire shaft with water source blocks
Place kelp from the bottom all the way to the top of the shaft
Let the kelp grow to convert flowing water into source blocks
Break the kelp
Place soul sand at the bottom for an upward elevator
Place magma block at the bottom for a downward elevator
Add an opening at the top and bottom for entry and exit
Test the elevator by stepping into the water column