Not sure this is in the right category but anyway ...
In a few months I will be buying and installing computer parts and obviously one of them is going to be the heat sink. Say I put the thermal compound on and then the heatsink, wait ten seconds and then take the heatsink off to check if it spread evenly, can I put the heatsink back on without applying more thermal paste?
I know it sounds stupid but I have no idea and I kinda don't want to screw it up
No. If you remove the heat sink after being in contact with the thermal compound, you will need to clean both heat sink and CPU and then re-apply new thermal compound. Removing and then replacing the heat sink without applying new thermal compound could introduce air bubbles which do not transfer heat from the CPU to the heat sink.
Well here's my 2 cents, apply the small pea size doze of thermal paste on the bottom of the heat sink then using a plastic card or business card, spread the joy evenly, then slap the heat sink on the processor and dont look back. Dont take it off to "check". The business card spread is your magic touch, then assemble like a boss.
Lentil-sized dot-in-the-middle, without spreading, is the most foolproof method.
That said, if you take your CPU cooler off after you've applied and installed it, if you re-install it without removing and re-applying your thermal compound, you will trap a lot of air bubbles. Your computer will still work, but temperatures will be suboptimal than what they could be.
Putting a dot of paste in the center of your heat spreader is a bad idea. Heat spreaders are square and the squeezed out paste pattern will be round with no paste in the corners. Now, someone will be along shortly to tell you how wrong I am because most of the heat is generated in the center. Notice that they will say "most". Just because the center is hotter is no reason to ignore the corners. If you do the math you'll find that using the circular method reduces the thermal path by over 21%. The proper way to do this is to spread a thin even layer over the complete heat spreader. A credit card works well. It may take a little practice to get it right but it's worth the effort.