I recommend going to Richard Binder's site and read 'replacing a sac' and the rest of his reference stuff.
http://www.richardspens.com/?page=main.htmIf you are just planing on replacing a few sacs, I recommend a set of section pliers, the 'plug pliers that are sold at auto stores for ~$10 here work well, a selection of sacs and some shellac. A heat source like a hair-dryer may be necessary. Heat should be used carefully and you need some way to check the temperature. I use a sac spreader one time ... then figured it was a PITA and just use my fingers.
Before using a heat gun, try to learn what about 140-150F temperature feels like. you will be close enough for most pen repairs when you learn that. Check your 'reference temperature' often until you are confident you aren't cooking your pens. Be gentle and find some old junk pens to practice your torture skills on before you take them to something that will break your heart when you ruin it.
And, do get 'Da Book'. It isn't perfect, there is a lot of just plain wrong information in it and it is the best reference available. The two main wrong things are the use of an alcohol lamp ... there are just better methods now, and Frank's use of nail polish. His descriptions of how to get stuff apart is necessary at times.
Ron