For my Engineering IV class, I took apart an old 1949 Westinghouse radio and converted it into a new bluetooth stereo with audio-synced lights.
Step 1: Opening Up and Cleaning Out the Radio
Take off the back or bottom panel and take a look inside the cabinet- more than likely, it's full of dust and dirt. Snip any long wires and find where the electronics are contained. Once you've found a way to get it out of the cabinet, you'll likely have a lot of cleaning to do. Personally, I found that windex and a microfiber cloth works pretty well in removing most of the grime inside the cabinet. Most of the work for this step is removing the electronics themselves. Look for rivets and screws first, then snip wires, then use pliers or a flathead screwdriver to pry out everything that's left. This would be a good time to take measurements and get a good picture of how things will have to fit inside the cabinet.
Step 2: Research and Planning
The next step is most important, and the most individual. Decide what kinds of things you want for your speaker. Personally, I wanted powerful audio and lights to sync up with the music. The exact models of speaker, lights, and bluetooth board depends entirely on how you want your speaker to perform. I recommend doing your own research before purchasing or deciding anything. Personally, I went for two full-range coaxial speakers from Kicker. I bought this bluetooth module/amp from Amazon. I also purchased some sound-controlled LED light strips and a power supply to control everything. One roadblock I ran into here was that I failed to pick a power supply strong enough to support the speakers and lights- I had to order and wait for another one. After you figure out your electronics, you need to design the enclosure for your speakers. Good resources for designing enclosures can be found here and here. Keep in mind the physical constraints of the cabinet and the original layout when designing an enclosure. My radio had a pre-existing hole for the original speaker, so I designed my enclosure to direct two speakers towards that hole.
Step 3: Building Your Enclosure
From what I've found on the internet, the best material for speaker enclosures is 3/4" MDF. There's not much to explain here, you just have to build what you've designed. Personally, I built my enclosure using a table saw to cut the sides pieces, a chop saw for the angle cuts, and wood glue/clamps to put it together. This is easily the most time consuming step of the whole process. Keep in mind the limitations of your radio cabinet as you build, and check it all before you put it together to make sure that it will all fit. Again, I had to deal with this as my enclosure ended up being just slightly too big, and I had to sand it down and carefully set it in there.
Step 4: Test Your Electronics
Before you install anything, you need to test it all. Wire everything together, securing it with masking tape, and plug it in. If there are any problems, you'll have to figure them out now. Make sure your wires are long enough to reach when you install them.
Step 5: Solder and Install
Once you've tested everything and are sure your setup works, it's time to install everything. Solder the connections and put everything in the cabinet. I found that I was able to install my bluetooth board inside the old electronics enclosure, and put the power supply inside one of the side compartments. Initially, I had planned to install it behind the speaker enclosure, but I didn't realize how much space the enclosure took up- which is why it is very important to consider all elements when designing your setup. I also had to drill a hole in the front to install my on/off switch.
Step 6: Finish Aesthetic and Complete!
Sand and add a finish or repaint your radio, and make panels to cover the back or any visible electronics. Finish anything else you need to do to make the radio look the way you want it to. [This is for a school project that had to be published- but it's not fully done yet. Check in later to see the full finished writeup.]