The first synthesizers looked nothing like the synthesizers you can buy nowadays. They were very big and were made of separate modules. One of the biggest names was Moog Music that build these kind of synthesizers from 1967. But they were very very expensive. And even if you would find a system like a Moog-55 nowadays it would still cost a lot. So in time people started cloning these modules and made their own modules as well. Luckily the DIY (Do It Yourself) community is very open and a lot of people share their schematics and designs on public websites and forums. Using this information I'm currently building a very big 70's Moog like synthesizer myself. I also bought some ready-to-go modules from Synthesizers.com (dotcom), but most of the stuff I'm building myself from scratch.
The first modules I build were designed by Yves Usson. A Frenchman that runs a great website called YuSynth.net. He has a lot of designs on there. I started the big modular project by building a lot of his modules. In the picture on the right you can see just on such a module from the back. Making a module like this all starts with making a PCB. That is an epoxy board with copper lanes that has holes in them. Through the holes you stick the components like resistors, transistors etc. And then you solder the components to the copper on the PCB. In this way the components are interconnected. Then you make a front panel to hold the connectors and knobs. Then you need a way to fasten everything together using some kind of bracket and finally wire the PCB to the front panel components.
When everything is done you can start testing and calibrating the modules. You do need some special equipment for that like an oscilloscope, frequency counter, and multimeter. Here you can also see the front of the modules. You can see that they all interconnect using big jack patch cables. There are two kind of signals throughout the synthesizer. Audio signals and Control Voltages (CV). These signals are all 0-10 Volt signals. All modules use the same standard. Even for tuning they all do one Octave per Volt. In this way you can connect all these modules together independent of brand. This standard was set by Moog and is actually still used in modern modular synthesizer so that they can still connect to the old stuff too. Well in the mean time I'm building a lot of different kinds of modules. You can learn all about them and how they were made in the menu on the left.