It isn't too bad, unless you're doing something like this:
Woah boy that one wore me out!
This is how I did it, and I would say that it isn't too hard:
1) Use razor blades and scrape away as much of the old silicone as you can. Silicone doesn't stick to silicone very well, so you'll want a clean surface to seal. Feel with your fingers to make sure you didn't miss any little boogery bits of silicone.
2) Wipe it down to get rid of dust on the inside.
3) If you want neat lines, tape it off like I did in the picture above. Nice, but not necessary.
4) Go to the hardware store and buy clear 100% silicone in the tubes and a gun to dispense it with. Avoid the stuff that has a mildew retardant in it, just find the stuff that says pure 100% silicone. GE brand is what I used.
5) Buy latex gloves while you're at it.
6) Turn on a ceiling fan and open the windows - this stuff stinks like vinegar and will make you dizzy, especially if you have your head inside an aquarium! Put a bead of silicone down each joint where glass comes together and then smooth it down with your finger (wear the gloves - it does NOT come off skin very easily!).
7) Remove the tape while it is still kinda wet (so you don't pull up the cured silicone) and let it cure for at least 24 hours - if not longer - before putting water in it.
here's where i learned to reseal my 150g tank. http://aquamaniacs.net/forum/cms_view_article.php?aid=107
It's not that difficult. I used GE silicone I at Lowes, which is pretty cheap. if you follow the link direction, then there' shouldn't be any problem. There's this girl name Kath, member of this forum, helped me tremendously also.
word of advice, when smooth the seam, i recommend using a piece of cardboard rather your fingers. bend it like a bow and smooth it out. This make it look really professional. if you need any help PM me. I recently resealed my 150g tank.