When you’re crocheting rows of afghan stitch (also called Tunisian Simple Stitch), you typically work each new stitch by inserting your crochet hook into the vertical bars, wrapping the yarn around your hook and pulling it through to create a new loop.
When you want to increase, you’ll have to sneak a stitch into a different spot. There are many possibilities for doing this — but one of the most logical and obvious places to add a stitch is the horizontal spot at the top of your work, in between the two vertical bars. Insert your hook into that spot, pull up a loop and then continue working into the rest of the horizontal bars across the row. Then work your return pass as usual.
That’s it! It’s easy to work an increase in afghan stitch; there’s really nothing to it. Happy increasing!