After you remove the original counter top it leave the top of the cabinet open.
Then we built a box with a heavy plywood bottom about 3/4″ deep (depending on your vanity and plumbing).
Attached the painted box to the inside of the cabinet and fill with pebbles (I used polished)
Measure the glass you will need.
Be sure it’s thick enough to hold the sink and have the glass company cut holes for the sink and faucet.
Apply the glass and install the sink and faucet.