Now that you've more or less found the pipe, it's probably best to reveal it by going to the surface and just digging out a shaft straight down. As stated earlier, once you get a down stair into the pipe, it will reveal the whole thing. Nothing can get out of the pipe through this shaft because there's no up stairs on the magma's surface, but I like to cap it off with a constructed floor.
The easiest way to tap the magma is to find a place in the pipe where the upper level is smaller than the lower level. Dig the magma tunnel that your forges and smelters are going to be above, and dig it to two tiles away from the magma. Smooth and engrave a fortification at the end of the tunnel (the fortification will have one tile of obsidian between it and the lava). Then get everyone out of the tunnel, and channel out that one tile from above. The Fortification will keep most of the magma pipe beasties out - flow can push stuff through impassable tiles (like fortifications), but maga doesn't flow very well, so the odds of having something sucked through the fortification is very, very low.
Since magma doesn't flow well, it does take forever to get your magma to 4/7+ (depending on length, it can take several years for the whole thing to fill up).
Recommendation: Do not make magma buildings until it's always 5/7. This is because the moment it dips below 4/7 (even if it's just for one step) it removes all pending jobs (annoying), and any moody dwarf that has chosen that particular forge will go instantly insane (very annoying).
Visuals! (X = unmined obsidian, F = fortification, M = magma, C = channel here)
Side view:
XXXXXMMMMM
XXXXMMMMMM
This is what you're looking for when you go to make your magma tunnel.
___CXMMMM
__FCMMMMM
your miner stands up top and channels away, perfectly safe.
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You can either wall up this access tunnel or just floor over the channel, either way, magma beasties can't get through. Magma will flood through the lower tunnel, but it won't ever go up into the upper tunnel (well, unless you use a pump to pressurize the magma, but if you're doing that, you don't need a basic tutorial)
One thing you can do to speed up the usable magma flow (getting to and keeping it at 4/7) without having to use pumps is to make your magma tunnel fairly short, and make a whole bunch of them in a row, with one tile of unmined rock between them. When the first segment fills up to 7/7, channel out the block. If you make them short enough, then even when it's filling up the new segment, the old segments won't drop below 7/7. More micromanagement, but you can use it earlier. If you have a ton of bauxite, you could do it automatically with pressure plates at the end of each segment opening up bauxite floodgates. It's more work up front, but you don't have to pay attention to it once it's going, which is a plus.