I am using UK crochet terms and this pillow was made from treble stitches in the back loop (double crochet stitches US terms). There are different ways of making a straight edge when working with a treble crochet stitch pattern. This is how I do it and I carefully count my stitches every other row or so to make sure I am not making mistakes.
How to crochet a straight edge/side tutorial
When you have made your last treble in the last stitch you finish off your row with a treble in the top stitch (3rd chain) of the first treble you made in your previous row.
Chain 3 and turn your work.
Skip the first stitch and make your next treble in the 2nd stitch. You can see I have marked the right stitch to start in with my tapestry needle.
Make a treble. For this pattern I am working only in the back loop of the stitch as you can tell, it looks a little bit different from a normal stitch. Now you just continue to make your trebles in every stitch until you reach the end of the row and then you start all over from Picture 1.
This way you won't get gaps at the edge/side. It is even and neat, although my large picture shows a bit of a wobbly edge... Hmmm... I blame it on the stone table and that I didn't smooth out my crochet piece before taking the picture (or maybe I failed with some stitches there - no one will ever know - Twink!!). :D
Good luck, and if you do miss a stitch in one row it is not the end of the world, right? You can always correct it in your next row unless you are WAAAY far out with your stitches. :D Keep counting. I hope this was helpful.