Use a shortened version of the title, unless the title fits within the space limit. The running head should include 50 characters or less, to ensure it doesn't take up too much space in the margin. Use the first several significant words of the title. For example, the APA manual demonstrates, a paper titled "Inhibitory Influences on Asychrony as a Cue for Auditory Segregation" could have this running title: "Inhibitory Influences on Asychrony."
On the first page, write the phrase "Running head:" -- including the colon -- immediately before the running head itself. This identifies the running head, clarifying that it's only a short version of the title. Capitalize just the first letter. Only include this identifier on the first page; on all following pages, omit it.
Write the running head in all capital letters -- for example, "INHIBITORY INFLUENCES ON ASYCHRONY." This helps visually separate the running head from the main body of text on the page.
Put the running head in the top margin, aligned flush left -- opposite the page numbers. Use your word processor's automatic function to insert the text in the header; don't type it at the top of each page, and don't add it manually after printing. This ensures that the running head remains the same on each page and remains separated from the main text.