All my desktops are DIY. I usually buy the motherboard, CPU and RAM(if no onboard video, then need to get GPU) from the same shop. Then tell the shop that you want to them to do power test i.e. assemble the cpu, RAM modules and GPU(if no onboard video) and power up. You verify your RAM size and cpu model & clock speed. Don't disassemble. Just bring these assembled parts home. Must nego this for free.
Example of standoff.
Align the standoff to the motherboard screw hole.
Don't forget to install the backplate that comes with the motherboard.