Clean out everything, get measurements, and prepare with a list of tools and objects. Take into consideration chicken math and make sure that you have everything you need for your flock. Here are some of my suggestions and things to watch out for when building.
I would first make sure that the three sides you are using are secure and will not allow any predators in. Then check for rotten/moldy wood. If there is rotten/moldy wood, take out those planks and replace them. I would place a layer of plywood over these planks for protection and to allow for insulation(keeping the metal on the outside for protection as well). Use the wall farthest away from the door for your egg boxes and place the roosts on the other walls above the top of your nest boxes. Do not allow your chickens to roost on or next to the nest boxes.
Check the roof for leaks. If there are any leaks, fix them up immediately. If there are any moldy/rotten wood planks, replace them. I would go with a metal roof or at least something that doesn't trap moisture because rain comes from the sky and will often hit the roof, not the walls.
Make a new floor with wood planks and plywood. I recommend vinyl flooring to go over the plywood because it will be easy to clean and will not rot. Wood chips can be placed over the vinyl but it is not necessary. Be sure to raise the floor up a good 2-6" so that any little critters can't just dig straight in to the plywood.
To make the new wall, get wood planks, plywood, and Hard wire cloth. You have an opportunity to make a very nice ventilation wall. First build the wall itself with the door for you and your chickens separated. Once the wall is finished, cut out some windows. Use hinges and the left over piece of plywood to make shutters. But first place the Hard wire cloth over the windows to prevent predators. Make sure that you are able to lock the shutters snugly when winter comes and to keep out pests.
The door for you should be big enough to allow you to enter inside comfortably. Make sure that you have no holes big enough for critters and a lock to secure it. You can place more ventilation windows on the door if you want. The door for your chickens needs to be a certain size based upon what you want to use to close the door for the night. I recommend an automatic chicken door. The door also needs 1' or more off the ground to keep out critters during the day. Make sure that you have a chicken ladder coming from the ground to the door (may need one inside depending on how tall your floor is).
These are just suggestions, not instructions. Build your coop how you like.
