For buttercream, I place the supports in all but the top tier. Put the bottom tier on the cake base,Then stick all tiers in the freezer for about an hour. Pull out the bottom tier and the next tier up and stack. Pull out the next tier and stack, etc. Drive your dowel thru all tiers (to prevent tiers slipping). Decorate. If buttercream gets soft and it's difficult to decorate, into the refrigerator it goes.
For fondant, I cover the tiers the night before, place supports in, then refrigerate. The next day, pull the bottom tier out and start stacking as described above. When the fondant is cold, it is fairly firm and you are less likely to make mess when stacking.