If the OP has purchased a new 105 crankset then it's probably the external bearing bottom bracket type. If the Ultegra cranks are also the Shimano external BB bearing Hollowtech type you will need a 5mm allen key and not a huge amount of torque - just enough to overcome the 12-15Nm it was torqued up to.
First remove the chain (since you are going to need to shorten it. Otherwise you wouldn't need to remove it). You will then need the plastic tool to unscrew the cap on the non-drive crank arm. (You may have got one included for free with the new 105 crankset). Unscrew that. Then loosen both 5mm crank bolts. Unscrew and completely remove the outer of the two crank bolts. With a small screwdriver or similar, flip up the little black metal plate between the crack on the non-drive crank (it pivots on the back crank bolt). Keeping the plate up, gently pull the non-drive crank off. Then you can push the crank axle through towards the drive side (it's all one piece with the drive crank). If it sticks part way through, ensure it isn't catching on something, then tap the end of the axle with a block of wood to encourage it.
Installation is really the reverse. Push the new crank axle through from the drive side (assuming you are keeping the old BB), grease the splines, push on the non-drive side crank arm, ensuring the little metal plate sits down correctly, screw on the cap on the crank to pull the crank arm fully onto the axle (finger-tight is fine), then alternately tighten the two crank bolts a little at a time. Torque them up to 15Nm which is a pretty hard turn on a 3" allen key, not quite so hard on a longer key. Double check tightness (and again after you ride it a bit)
Lower the derailleur height as Retro Grouch said, so it is just above the new (smaller) 50T chain ring. This will screw up the adjustment of the FD cable so you will have to adjust the cable to compensate.
Shimano have downloadable PDFs with instructions for each crankset type on their website. Look under Cycling/Technical Documentation/Road Bike/105 (Ultegra is identical). Click Here
In all seriousness changing the cranks is easy. You will probably do more cursing shortening the chain and getting the FD adjustment right.
Last edited by scirocco; 11-06-08 at 05:15 AM.