If there's a jagged-tooth type mounting bracket attached to the back of the frame or an existing hole at the top of the frame, measure the distance between the center of the frame to the bracket or hole. Move your tape up that distance. Hammer in the fastener or hanging hardware and use the jagged teeth bracket and a small hand level to adjust the picture as needed to make the top of the frame even. The best way to use the level is lay it on the middle of the frame and adjust until the “bubble" appears in the middle of the level. Release and back away.
If your object has a wire stretching across the back of the frame, use your finger to apply tension to the wire, measure the distance from your finger to the middle of the frame, move the tape up that distance and pound in the fastener or hanging hardware. Use your level to fine-tune the hang by sliding the object up or down on the wire as needed.