Since flat volcanos are rare nowadays i'll go with my favorite for true volcanos.
Tapping the keg.
Dig out where you want your magma forges, put a stair down tucked in a corner somewhere. Under your forges build a radiator, thats the grid of 1-wide tunnels to fill and fuel the forges. check spacing to fit the number of forges you want in the right places, modifying these is essentially impossible once you fill.
Now dig a nice 1 wide tunnel all the way to 1 tile short of the volcano. If you want to theoretically be able to empty the radiator one day go ahead and install a lever-linked floodgate in magmasafe materials (mechanisms too remember). If you close the floodgate permenantly the radiator will actually eventually drop to the point you get brownouts, it needs topping up once every few years. With it shut you could pump the magma in the radiator elsewhere to do maint...try and avoid this.
I usually breach using an engraver since i like my miners and generally tap a volcano too early to even contemplate possible pick loss. Smooth the final obsidian tile, then set it to be fortified. As soon as the guy starts to cut fortifications pause, set a single smooth designate up in your magma forge and turn off any other engravers, now your guy should beeline for safety....it helps if he has good agi.
Once its filling, floor over the down stair you used to cut it. cut channels over the radiator grid and build workshops
The fortifications is to keep out magma critters btw, though i rarely have issues with mine liking to lurk by the sea.