AHH! How to add on!?!

I am making a crop top sweater that has panels and buttons in the back. I have finished the first panel and I am to “add on 15 sts” but I don’t know how to do so. I know how to increase by knitting to the front and back, but I don’t think that will work fifteen times… PLEASE HELP!!
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You basically work a cast on. Look at Amy’s instructions for Knit or Cable cast on

When you get to the point where you need to add stitches, just work one of these methods until you’ve added the needed number of stitches.