Nope, no science involved. Just 4 bolts per sieve. If they've been in for a while, the top one will take a little prying. Once you're done with your patch work, I'd make sure and take a little time to inspect the area of the chaffer frame where the chaffer bolts up. They are known for cracking out there and the chaffer will drop down and make a mess. Also a good idea while both sieves are out to set yourself down in the machine and look at the sawback grain pan. Even if you keep good bushings, the pan can crack apart and welding it wont get you by. I dont mean to ramble on, you probably allready know those things, but I'd hate to hear about you having those problems.
If the prying doesn't work a come along to something solid (even the ladder is all it need sometimes) and to the top sieve is all you need to get it moving. Watch your fingers when you take out the long bolts, since the sieve drops down and it can hurt. Just check everything for cracks since its all apart.