Measure Twice, Nail Once
Step 1: Arrange your assortment of pictures and wall hangings on the floor exactly as you want them and directly in front of the wall designated for hanging. This method will ensure that your grouping won't be too big or small for the proposed space.
Step 2: Make a sketch of the arrangement (don't fret about your drawing abilities); then measure and write down the desired spacing between the pieces.
Step 3: Depending upon your chosen arrangement, which may result in the pieces sharing a common top or bottom alignment (or it could be random), get someone to hold the largest or most defining item on the wall where you like it.
Step 4: Mark the top edge of the piece by centrally placing a self-stick note on the wall. Next, take that same picture or object, and turn it over to see its hanging attachment. If there's a picture wire, hold the wire taut in the center (or if you plan on installing two hangers, then use two fingers spaced about 4 inches from the center). Measure the top of the wire to the top of the frame, and write this distance down.
Step 5: Next, take this recorded number, and measure down from the self-stick note. Place another note somewhere within this field, and make a mark. (If using two hangers, remember to place notes on two equally spaced marks to the right and left of center.) Then nail or drill directly through this self-stick note, and peel it off to remove.
Step 6: With the guide picture or object positioned and hung, you can then arrange the other objects with the same method. Don't forget to incorporate the desired spacing as noted on your sketch.