The tutorial Kat gave is great!
If you choose to download Sims3Pack sims, make sure that you also check that they are not corrupt. Corrupt sims can really mess up your game! That's not a problem at this forum, but I can't vouch for every blog or other site around the net (especially the exchange - be careful with future downloads)
One of our members, Dachs, wrote a nice tutorial about that on her blog: