Creating the Base
One of the easiest things to do is create a reusable diamond base for creating harlequin design patterns over and over. By creating this base, you make it easy to simply click and fill in your patterns with whatever you would like without having to create a new base every time. The easiest way to do that is take advantage of the pattern fill option in Photoshop, and I'll show you how to do just that!
Create a new document 50 pixels wide by 100 pixels wide. This will give you the classic elongated diamonds that are part of the harlequin style. Ideally you will want to do this on a transparent canvas (no background) but for ease of viewing, I'm going to use a white background so you can see my lines very clearly.
Now we're going to need a little guidance here, so this is where we set up Photoshop's grid to give us a little bonus help. Simply go up to Edit > Preferences > Guides, Grid & Slices. Your window should look similar to this.
Now you simply have to change the options in grid to:
Gridlines every: 50 percent
Press OK when finished.
Now go to View > Extras > Grid. Your canvas should now look like this!
Zoom into your canvas a decent amount - trust me on this one, you're going to want to see as much as you can. I've zoomed in to about 800%. Here's what it looks like compared to the rest of my workspace.
Now grab yourself your line tool - it shares the same tab as your custom shape tool and your shape tools - set the line width to 2px, and begin to create a diamond shape. I'll show you the easiest way to do that with this quick diagram.
Simply drag your line tool from area to area, making sure to pay attention to the three specific key areas when you do so:
Now you have a completed diamonds that should look similar to this!
Now to define it as a pattern, simply go up to Edit > Define Pattern... It will open a window and ask you to save your pattern. Name it something like "diamond" or "harlequin pattern" -- organization counts, after all.