VELCRO may not be very glamorous, but it is very strong, inexpensive and is generally fairly easy for most people to open and close. I consider it a sewing staple.
Since you can't pin VELCRO in place (or at least it is very difficult), it is easy for it to get slightly off kilter when you sew it on. Any change in the position of the piece will probably mean it ends up a little crooked.
So here are two things you can do to help sew it on straight, and one extra general tip.
1. Line up the piece and then using a fabric marker or chalk draw brackets around both corners at either the top of the bottom, or maybe even all four corners. Then when you are sewing you can make sure to hold the piece inside the guide brackets to keep it straight.
2. Use thread that matches the color of your VELCRO. I know you may not want to slow down to change thread, but I promise it will help with how your finished piece looks. I often use contrast thread when making a pattern with VELCRO so people can see the stitching, but if I'm making a finished item to sell, I change my thread color.
The reason you want to use matching thread is because the contrast between the VELCRO, which is often black, and the thread makes every stitching imperfection highly visible. Not only is it hard to sew on the VELCRO piece so it is straight, it is also hard to sew a perfect square around it. Some imperfections in stitching aren't a big deal, but here is the problem: when you look at it, the first thing your eye will see is the wonky stitching. The VELCRO will appear to be sewn on crooked--even if it isn't.
3. When sewing on the hook side, try to sew between the rows of hooks. VELCRO's generic name is hook & loop tape, but it will always be VELCRO to me. The hook side is the rough side with all the teeth. The loop side is the softer, fuzzy side.
Many times when I'm sewing the sides of the hook half my thread breaks, which is so annoying. I've found that if I can sew between the rows of hooks, my thread is far less likely to break. Since it can be really hard to see the rows, this can be a pretty hard feat to manage sometimes, but it is worth trying to do.