I have done mostly fondant cakes which for the most part are easy to stack. When I tried to do a buttercream tiered cake it came out a mess, I am guessing that they should be frozen when you stake them. I know this is probably basic but any advise, do you eyeball where it goes or do you mark where it should go? I always leave finger marks at the bottom of the cakes when I stake them too, how do I avoid that?

I only make buttercream frosted cakes, and I never freeze them. I use the cake dividers to mark out where the tiers go, and to also put in the wooden dowels for support. I stack at the site, and add the bottom borders of frosting after I've stacked, so any fingermarks can be fixed and then covered over.

Stick the cakes in the fridge for 10-15 minutes before stacking. I always set a cake board the size of the cake that I am going to stack on the lower tier and then take my ruler and measure all the way around to make sure it is center. Once it is center, press down gently on the board so you make an imprint on the lower tier to use as a guide for where your cake should go. Then remeasure with the cake in place to make sure that it is centered. You should be able to clean up the fingerprint marks by gently rubbing a Viva paper towel over the marks. HTH & good luck!

I learned to stack from tonedna1. Here's her tutorial on stacking. Its made with a crusting BC. I have never had any issues with stacking this way. Hope this helps!

I wrote this in another thread:

One hand on the spatula and one hand underneath the cake (on its own cake circle) supporting the weight.

If you're using SPS, you can lower one side of the tier onto the plate, remove your hand, slide the tier into place across the plate with your spatula, lower the cake the rest of the way, and remove your spatula.

If you aren't using SPS, it helps with positioning if you're placing your tier onto another cake circle (just as you would with a single plate separator). So rather than trying to lower it on top of a buttercream surface (while denting the lower cake, trying to get your hand and spatula out without making a mess, and possibly disturbing the dowels), you're lowering it onto a corrugated cake circle instead that's already stuck in place on the surface of the lower tier. Doubling up on circles is easier than risking damage and causing internal support issues, and you'll still be able to hammer a center dowel through all tiers without a problem.

Here's the thread:
http://cakecentral.com/cake-decorating-ftopict-710889-fingers.html
Hope it helps.