I get magma in the first year through a simple digging to victory strategy: bore a 1x1 up/down staircase in 3x3 landing rooms straight down. When/if you breach caverns, seal them IMMEDIATLEY with walls, hatch covers, or constructed up stairs as you are able. When a digging designation cancels for warm stone, you're on magma: go up a level and dig on top of where the designation canceled, and all ajacent squares: if you're stopped by warm stone at any point, go up and do it again. If you aren't stopped by warm stone, the uppermost warm stone layer is ABOVE magma, not next to it: dig through THAT warm stone, and channel a square that was formerly warm. This will get you a glimpse of your magma source, so you know its borders.
Pick two adjacent levels, at least the lower of which has your magma source. On the lower level, dig rooms for as many magma workshops as you think you'll need, remembering that the blocked square of most (all the furnaces) is in the north. On the level above, dig the EXACT SAME LAYOUT. Channel the squares that will be non-walkable for the workshops you want to build, and build the magma workshops on the upper level. they will be unpowered. Surround your staircase on the lower level with constructions and one magma-safe door facing the magma. Your workshop blueprint should be between the stairs and the magma. On the lower level, dig passages so all the workshops are connected, and a passage to the magma. Don't break through the last wall to magma yet: first clear all objects from this floor, including stone, by dumping/quantum stockpile. You may want one close by on the upper level.
Once the lower level is clear, it's time to power your magma industries. turn off mining on all dwarves except one junk dwarf (most recently I picked a fresh migrant who claimed novice strand extractor and high master liar), and have your masons make a blank slab: whatever unlucky soul breaches the magma has, at best, a 50% chance at survival. Designate the last tile separating your lower blueprint from the magma source dug.
As soon as mr. junky starts digging, pause. Profile a lever somewhere in the fortress to him (it doesn't even have to do anything) and order it pulled. This might make him a couple ticks quicker to run. When the magma is breached, it's a race between the junk dwarf and it for the magma-safe door to the stairs. If the dwarf makes it, he lives and you get your pick back. Otherwise, mourn the loss of the pick and engrave the slab. For extra points, place the slab somewhere in your magma industries. Construct a wall that blocks off the access/escape door so building destroyers can't mess you up, and you've done it: Safe, powered magma industries in record time.
You don't have to build all your magma workshops before breaching magma, but if you can't build it, don't channel its room yet, as that provides a clear shot for magma creatures to enter your fort. only open up access to magma RIGHT before you build your shiny new magma workshop. In particular, you'll probably have several blank rooms set aside for magma forges, as you won't have the anvils when you first breach to build more than one or two.
While you can extend this setup by building another template and breaching magma again, that might cause your old magma workshops to "flicker" as the new area fills, and it requires another suicidal magma breach, so build the first one big. A good size for your metal industry would be 9 magma smelters and 6 magma forges (15 workshops) giving you an edge on smelting and some room to not be disrupted when a mood claims a magma forge. The remainder depends on how much you like glass and pottery: any number of magma glass furnaces can be supported as long as you have one (prefrably non-magma) glass furnace set to gather sand/r for every 4 furnaces making glass. If you want to make clear glass, you ALSO need (by my estimation) 3-4 wood furnaces, 3 asherys, and 3 magma kilns for each 2 magma glass furnaces making clear glass. I haven't played with the pottery industry much, but 4 magma kilns for actually making things with a regular one just gathering clay sounds like good proportions, same as green glass. All in all, have room for a good 20-30 workshops if you want to never have to do it again, a dozen or so if you don't mind a repeat performance.
EDIT: okay, that probably doesn't sound convenient, but trust me it's a lot easier than it sounds spelling out every step. barring bad luck with the exploratory mining, you get ACTIVE magma smelters and forge(s) by the first winter, without much disruption to your normal industries.