Dig hallways ending with downward ramps until I hit the first stone layer, then set up a farmng area in the first underground level by tapping a pond. Then I carve out a dining room, a workshop area, a space for a trade depot (the trade room is 7x7 and stockpiles for goods go in a C-shaped room around it,) and a dormitory because dwarves still won't use thier own rooms yet. Once I've got some quarry bushes bagged, or some meat, fish, and cheese from trading, I dig out a room for a kitchen where I store the perishable items until they can be cooked (in case they rot before I can cook them, so miasma doesn't spread too much.) The living areas here are fairly compact and utilitarian aside from decorations for the dining room and the leaderr's office.
THEN I get to work digging out dedicated storage areas. Once all that's done, I dig down to the magma and start smelting ores and training my smiths, as well as start turning out metal barrels and bins. Once I have two hundred serrated metal (or glass) disks, I dig a 2 tile wide hallway into the caverns, fill the first twenty spaces (10x2) with traps, and stick some dogs on ropes behind them to lure in underground life. Once I have some decent equipment, in case something gets past the traps somehow, I set up a loom just behind the dogs and tell a weaver to get to work collecting webs.
Somewhere in there I dig out as much of the soil layers as I can so I can convert them into farms for natural underground plant life, so I can chop trees and gather underground plants without exposing my dwarves to the dangers of the caverns unneccesarily.
Once all THAT'S done, I get to work walling off the caverns.