Use a Piece of Wood Trim to Make Hanging Heavy Items Easier
When hanging heavy objects like large mirrors or framed art, you need enough support to hold them securely against the wall, and using two hooks is the best way to do this, but hanging these hooks first and then the art can be tricky. … Use a piece of wood to make sure the hooks are at the correct distance from each other. Here’s how.
You will need a piece of wood with a straight edge on one side and six or more inches long to leave enough space between the hooks. Mark the center of the tree, then place it under the wire attached to the back of the frame. Measure from the center mark on the tree to the top of the frame.
Mark on the wall where you want the top of the frame to hit, then use the distance you just measured to mark where the hooks should sit in the wall. Keep the wood plank flat and aligned with this mark, then mark where the ends of the plank meet the wall. Nail a hook to each of these marks and hang the frame. See the video above for step-by-step visual instructions, starting at 02:15.
How to hang a heavy mirror “wiki useful This old house (YouTube)