Gather blocks
Gather water buckets
Gather kelp
Gather soul sand
Gather signs or trapdoors
Build a vertical shaft
Place soul sand at the bottom
Place water source blocks in the shaft
Plant kelp from bottom to top
Let the kelp turn flowing water into source blocks
Break the kelp
Place signs or trapdoors if needed to contain water
Use the bubble column to go up
Replace soul sand with magma block if you want a downward elevator