First, you need to make a platform for the condo. The platform is just a square piece of plywood. You will have to select the width of the platform according to the height you select. The higher the condo is placed; the platform should be raised accordingly.
A house on the main platform is a great way to add additional weight and play area. Simply cut five sides for the house of plywood, four for the sides and one for the top. Use a handsaw or band saw to cut an arched doorway into one of the sides. Screw the sides together with the short screws and then attach it to the base. Make sure to leave enough room on the platform to attach your 2x4's for the other platforms. Use carpet to cover the house if desired.
Using the 2x4's, choose the height of the other platforms you are planning to add (2 to 3 platforms is usually enough). Cut each 2x4 to the corresponding height that you want. Keep in mind the size of your cat so that your condo is not too big or small.
Now you need to build the additional platforms. The size of the platform should big enough to easily accommodate your cat. Cut carpet about one inch larger on all sides than the platforms. Stretch the carpet over the sides and staple to the underside.
Add the platforms to your 2x4's using 90 degree brackets. Make sure that the screws do not poke through the other side.
Attach 2x4's to the base. If you push them against the house, you can screw them to both the base and condo for added support. This is especially helpful if you have tall platforms. Use the long screws to attach the bottom of the platform up into the 2x4. You may also use 90 degree brackets if it feels unstable. Your cat condo is now built and it is time for the fun stuff.
Use binder twine to wrap around the 2x4's. Wrap tightly and keep the wrap close to itself. Secure at both ends with a staple gun. This creates a scratching post built right into the cat condo.
Use the staple gun to add any toys that your cat will enjoy. Attach them to the underside of a platform so that it dangles.