I have been doing the following for the last ten years when building PCs.
1: If you are using the stock heatsink fan, skip to step 2.
2: Lay a half folded bath towel on a sturdy table.
3: Lay the motherboards anti-static bag on the towel.
4:If you are using an aftermarket heatsink/fan that requires removing the stock retention bracket, remove it now.
5: Lay the motherboard on the anti-static bag (be sure to install the aftermarket heatsink back plate before laying it down), install the CPU, then apply the thermal paste.
6: Install heatsink/fan.
7: Install the RAM.
8: Install the motherboards I/O shield (plate) into the case.
9: Install the proper amount/height of motherboard stand-offs into the case.
10: Install motherboard into case using supplied screws and tighten firmly.
I use the table method because the back of the motherboard is completely supported by the table instead of only being supported by the stand-offs inside the case.
It especially helps with installing the RAM.
Good luck!