Some yokes are bonded to the tube and if thats the case with your donor its not going to come off all that easily. You should be able to losen that clamp on the back and firmly grab the yoke (best place is the large plastic area close to the bell of the tube) and just wiggle left and right while you pull backwards. If it starts to come off then your good to go, if not it might be bonded and your in for it.
You should reuse the magnetice rings that are with the donor tube. With a flet tip pen mark the rings position on the tube by drawing across the ring onto the glass in several spots. It will be easier for you to figure out where they go when your ready to put them back on.
As far as getting the yoke back on the new tube. If its out of square your picture will also be out of square. Trust me, you'll see what I'm talking about when you power it up. Just make sure that the pins on the top of the yoke are near 12 o'clock and you will be fine. If your careful you can twist the yoke while the monitor is powered up to get the picture straightened out.