Make sure you have followed the static safeguards before touching the motherboard or any circuit boards.
Most new motherboards will come in a plastic bag with a thin sheet of foam on the bottom of the board and the box should be a fairly sturdy container to protect it in shipping.
Remove the bag with the motherboard inside from the box and close the top of the box.
Remove the motherboard from the bag leaving the foam in place then place the plastic bag on top of the box then place the motherboard on top of the flattened out bag still resting on the thin foam.
This gives you several layers of static protection for your motherboard. Plus a soft yet firm place to protect the circuits on the underside of the motherboard when it's time to press components into place.
Locate the CPU socket (using the pictures in the owners manual that came with the motherboard if your not sure what it looks like.
There will be a metal or plastic lever on one side of the CPU socket. Open it up fully into the unlocked position so it's ready for the CPU placement.
Usually this lever has a small catch it's under and can be opened up by carefully pressing down on the lever slightly then gently pulling it away from the socket catch just until it clears so it will raise when you let go.
Do this carefully so as not to break the catch.
Then raise it up to it's full position which should be a ninety degree angle up from the socket
Check your owners manual for the exact method before you start the assembly process.
(just to be on the safe side in case you have a different style I haven't come across yet)
Carefully remove the CPU from the box it came in being careful not to touch any of the little prongs that stick out of the bottom. (there's dozens of them there)
Holding the CPU by two opposite edges using the thumb and one other finger (of your choosing) carefully place the CPU so the mark on the corner of the CPU is exactly over the similar mark on the CPU socket of the motherboard. Place the CPU just barely above the socket almost but not touching the motherboard or socket.
Try to align the little prongs on the bottom of the CPU with the dozens of corresponding holes in the socket. ( don't worry about being perfect just get as close to perfect as you can)
Holding the CPU just barely above the socket centered as close as you can and with the corner mark of the CPU lined up above the corresponding mark on the CPU socket let go of the CPU so it drops into the socket. It should easily fall into the socket without any other effort from you.
Place a finger on top of the CPU and lightly press down holding it in place as you return the locking lever to the locked position. This makes sure the CPU is well seated into the socket.