Okay, the closet is empty, the walls are finished and you're rearing to go. In my closets, I typically just use 1x12x8 pine boards for my shelving and cubes. The grade of wood is up to you. I go with a basic pine board that looks nice but isn't extra fancy. (It IS a closet after all.) The 1x12x8 typically runs me about $10 a board. (Just to give you an idea of how much your project would cost.)
Start by choosing your cube size and cutting your boards to length. Make sure to give yourself a clearance if you're building it first and installing second. Otherwise it wont fit in your closet after it's built. We generally allow 3 inches from the top of the ceiling.
After they are all cut, lay it out on a flat surface and put it together as it's laying on the floor. (Easier than building it standing.)
It helps to have a SPEED SQUARE to make sure the shelves are not going in crooked.
If available, use a BRAD NAILER to get it all together. Then go back through and pre-drill and add 4 screws to each shelf. (For extra support.)Once that's done, stand it up and make sure it slides easily into place.
It's now time to stain. (Or paint) It's best to stain or paint it before you anchor it to the walls. When choosing your finish remember that there will most likely be bins or baskets on your shelves. If you choose to paint your shelves, it is wise to go with an oil based primer first on them, then your paint of choice. And while it's pretty stinky, an oil based paint (or an oil based stain) will be the most durable.
Once it has the finish you like, you're almost done! After it's fully dried....anchor it to the walls with either L brackets or just screws. (Depending on the stud situation in your walls.)
Now it's time to hang your clothes bars. Depending on the clothes bar that you choose, you just want to make sure that the bracket that catches your bar is well anchored into a stud. if your bracket doesn't happen to fall on a stud in your wall, run a 1x4 the depth of the closet finding 2 studs in the board and attach your bracket to that.
You have now created a custom closet to fit YOUR needs....and for 1/4 of the price.