1. Take your time.
2. Consider following premade plans.If you're new to woodworking and DIY, find a plan in a book or online that goes through the steps in detail, so you can just follow along like it's a recipe for a cake. I have a step by step coop-building project that I did this way, and it went really well. This spring we will add a rectangular base about 4 feet high and top it with that coop to make a larger stationary coop.
3. Don’t overbuild.The coop needs to have a solid floor to keep raccoons and other predators out, unless you're using other means such as electric net fencing. And it needs to be sturdy. But it doesn't need to be Fort Knox, either. Remember that it is going to be completely covered in chicken poop within a month or two.
4. If making your own plans, write or draw them out in detail first.If you are a carpenter or naturally gifted with woodworking skills, you're not reading this. So take the time to think through your design as much as possible and write out the steps with little sketches of what your thoughts are. This will also help if you get interrupted and have to put aside the building project for a time, so you remember what you were thinking.
5. Use recycled materials whenever possible."Upcycling" materials - making something better out of used materials - is a great way to build a coop on the cheap. But make sure that you don't use worn, rotting wood, or low-quality materials that will end up wearing out quickly. And sometimes the irregularities of a scavenged window or other item will be too hard to work around. If you're a beginner, this can add up to a lot of frustration quickly, so use upcycled or recycled materials thoughtfully.
6. Size your chicken coop properly.
If your birds will have access to an outdoor run, you'll want to allow for a minimum of 2-3 square feet per bird inside the coop, and aim for about 4 square feet per bird in the run. The higher you can go, the better, though. If your birds will be cooped all winter (chickens don't like to go out onto snowy surfaces), allow for 5-10 square feet per chicken. For birds that will be completely confined in a chicken tractor without an outdoor pen, give a minimum of 5 square feet per bird.
These are just general guidelines. The bigger the chicken, the more space it needs - so meat birds in general require more space than laying hens, and full-grown pullets need more space than baby chicks. Most annoying chicken problems like pecking and aggressiveness can be cured with more space, so plan for as generously-sized a coop as you can fit or afford.