Place two vertical columns of blocks side by side
Leave a one-block-wide shaft for the elevator
Build the shaft from the bottom to the top
Place a door, sign, or trapdoor at the bottom to hold water in place
Fill the entire shaft with water source blocks
Place kelp from the bottom to the top of the water column
Let the kelp turn all water blocks 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
Enter the water column to ride the elevator
Add a roof or top exit to prevent falling out
Add a second shaft if you want both up and down travel
Use blocks around the shaft to keep the structure enclosed