How can I make glass tunnels through lava?

I have done this many times while building a bedrock-layer minecart track to connect my various dwellings. The strategy I have used with great success is to temporarily block the lava flow with a scaffolding of nether brick fence. Being made of stone, it will not catch fire and burn up like regular wood fencing, but it will still allow you to reach through it to place your glass blocks behind it.

In addition, the spaces in and around the fencing can give you enough room to place the glass against the sides of either a fence post on the edge of the scaffolding or a previously-placed block of glass. This means you can still build your glass tunnel even if you can't reach a nearby wall, floor, or ceiling of the lava pool to place the block against. This is very handy if you hit a lava pool that is very wide and/or deep.

It should also be noted that these same techniques are useful if you hit an underground pool of water, although in that case regular flammable fence would do just fine too.

It's not hard, if you're careful :) Careful includes having a bucket of water in your inventory, and/or a pool of water nearby - maybe a chest for those high-value items.

Lava moves slowly, so you can replace blocks touching it with glass before it reaches you. Make sure you know what keys your glass blocks are in :) Break the block, quickly place the glass.

Also make sure you have a line-of-sight on a non-lava block to click the glass block - if I recall right, you can't attach glass to lava. :)

If you are having speed problems, you can also temporarily replace a lava block with, say, gravel - then break the outer block, put in glass, and dig up the gravel block, letting the lava flow into the glass.

Me and a friend actually just did this as a new neato way to light the floor of a building we just made. The end result is awesome, the entire floor is glass and there's lava right below it that glows up through the building, but in the process we died like 5 times. So just be careful, have water buckets at the ready, dig the area all around the lava so you have some room, then build the glass in and fill around, be as careful as you can about not letting the lava leak out. Probably easier to co-ordinate in single player since you can't kill each other. hah.