This tutorial will show you how to make horizontal and vertical dashed lines.
Step 1
Create a new document 10 pixels wide and 1 pixel tall (or wider/thinner if you prefer) and choose the color you want your dashes to be. Using the Zoom Tool, get a close up of your document and draw a selection around all the pixels except for one on the right and one on the left. Press Option+Delete (PC: Alt+Delete) to fill the selection with your chosen color (see below zoomed-in example).
Step 2
Next press Cmd+A (PC: Ctrl+A) to Select All and then choose Edit » Define Pattern, which will define the selected area as a reusable pattern in Photoshop. Choose a name for your pattern, such as “dashed line” and press OK.
Step 3
Now that you’ve finished making your dashed line pattern, it’s time to apply it to whatever design you’d like. Either open up a preexisting image you’d like to add the line to, or create a new blank file to work with.
Make a new layer, and choose the Rectangular Marquee Tool.
Now, zoom in so you can get a closer look (well, unless your eyesight is a lot better than mine!) and then go ahead and draw a horizontal selection line on your image in the area you want your dashed line to appear. If you want a very thin dashed line, draw your selection to be 1-2px high… if you want a thicker dashed line, feel free to draw a taller selection. Your pattern will be “stacked” as many times as can fit into the height of your selection.
Go to Edit » Fill and choose Pattern from the dropdown—select your saved “dashed line” from the available Custom Patterns and press OK.
Step 3
You can also make vertical dashes with a few more steps. Firstly you could easily draw a horizontal line and then rotate the layer 90 degrees (Edit » Transform » Rotate 90° CW).
Otherwise, to make an actual vertical pattern you can reuse, return to your original 10x1 dash pattern image and flip it 90 degrees (Image » Rotate Canvas » 90° CW). Sample the image as a patten again and use this new pattern to fill your vertical selection on your new file or preexisting image.
Step 4
Yet another nifty idea to try is to make a circular selection instead of a line… give it a try and you’ll find yourself with a circle that’s been automatically filled with striped lines. Taking this yet another step further, try filling “any” shape (such as a star, triangle, etc). Have fun!