Running stitch is a super easy way to add a bit of color to a project, I like to add mine to knitted and crocheted projects with yarn leftovers. To begin, you'll need a tapestry/yarn needle, some yarn and the piece you'd like to embroider on. If you'd like to do this on fabric, just use embroidery floss and an embroidery needle instead.
Start by threading your needle with a tail as shown above and bring the needle up from back to front.
Using the stitches from your piece as a guideline (single crochets shown here), go over and under the stitches,
Next, pull the yarn through...running stitches!
Continue for as long as you like,
And for a neat finish, complete by ending with the yarn in the back. That's all there is to it!
Here's a video you can watch about the running stitch too: