Place two blocks vertically for the elevator base
Build a 1-block-wide shaft to the height you want
Hollow out the shaft so you can move inside it
Place water at the bottom of the shaft
Put kelp from the bottom water block all the way to the top
Let the kelp turn the water into source blocks
Break the kelp after the water is filled
Use soul sand under the bottom water block for an upward bubble elevator
Use magma block under the bottom water block for a downward bubble elevator
Surround the shaft with solid blocks to keep the water contained
Add signs or trapdoors at the top and bottom to stop water from spilling out
Enter the water column to move up or down
Add a landing platform at the top and bottom
Build a second shaft if you want both directions in one elevator
Label the entrances if needed