There are many styles of quail pen and no one right answer. I built the pen below without designs. Here are a few tips.
Most quail pens have a 1/2" galvanized wire mesh bottom so that the birds aren't walking in their own waste. I added a "litter box" drawer with wood pellets below the birds.
Make sure you can block off a small area with a second door the size of your arm for getting the birds out.
Consider how you'll keep the birds in this small area. This coop has a hinged door in the partition attached to a string that runs through the wire on the top of the pen.
Don't build the bird area too high. If the birds have enough room to get momentum, they'll hit their heads on the top and hurt themselves.