Go online, figure out the dimensions of the case you want. build a frame from square tubing(or thick dowels, SQUARE). aluminum preferably, steel second, wood if you cant get the first two. Decide how you want to mount your drives mobo, PSU, and fans. You can incorporate the mounting racks into the frame or construct them of sheet metal(or thin plywood/acrylic).
Next, what will you cover it with , or will you cover it at all? Again, use sheet metal(thin guage), wood, or acrylic(if you can get it). Cut into appropriate sized sheets to cover both sides, front, back, top, and bottom. Be sure and make your cutouts for your drives, PSU, fans, and mobo tray(which you should probably CAREFULLY cut out of your existing case.)before you assemble.
If a welder or a shop with one is available, pre assemble it, number the pieces to where each joint has a common number, make diagram of how it assembles and take it to the shop and have them weld it. Otherwise you can assemble the case with rivets and a pop rivet gun, machine screws, and flat right angle brackets which you can later polish. Or if you're using wood, finishing screws or nails.
Of course, if you can get acrylic by the sheet, make the whole thing out of acrylic!!!
That should be a pretty basic beginning. Check out the websites and case galleries for inspiration. You're not the first guy who didn't know what he was doing. We all were at some point!