So when I set out to fit some aftermarket seats to my 1989 BMW 325i, I decided to make my own brackets. The level of complexity can vary depending on whether or not you want to add adjustable slider brackets, etc..
My seats didn't have sliders, so I decided to add some aftermarket slider brackets, which upped the complexity, but I'll just go over the basic idea here.
-1/4" x 1-1/4" mild steel bar stock (~5-10' depending on your setup)
-measuring tools (you could get by with just a measuring tape, but something like a caliper comes in handy as well)
-metal working tools (bandsaw, chop saw, angle grinder, drill press, etc)
I built my brackets at the TechShop, in Menlo park, where they have all the tools and equipment I needed, but if all you have is a hacksaw and a cordless drill you could probably do everything but the welding. If you want more info about TechShop, check out their website, www.TechShop.ws
Step 1: Remove Your Current Seat
Step 2: Measure
Once you have the measurements, it helps to overlay them on top of each other to visualize how the two will fit together. I used Autodesk's AutoCAD Mechanical 2012 for this step, as it is readily available on computers at the TechShop.
Step 3: Design
Keep in mind when designing your brackets that you'll need to be able to assemble it, which means there needs to be clearance for bolt heads, nuts, wrenches, sockets, etc. I designed my brackets using Autodesk's AutoCAD at the TechShop, which made things a lot easier.
The two bolt patterns for my bracket were so close to each other that I had to notch the opposing sets of strips to make room for the bolts. I did this with an angle grinder and some patience.
Step 4: Fabricate
After testing the fitment, you're ready to weld. I usually put a few tack welds on there and then go back to recheck fitment before fully welding. I used the TIG welder at TechShop, but if you have a MIG welder at home that would work too.
Step 5: Finish/Install
Now install your finished bracket and enjoy your new seat!