I've done a little bit of digging into this and I'm afraid it's a bit difficult, even with substantial resources to put into the project. Making such a device is not impossible, but making it powerful enough to achieve your desired effects seems to be the sticking point. Your device will need 3 main subsystems. A pulsed power supply, a pulse forming network, and then some form of emitter. The power supply is actually quite easy - marx generators are the most suitable type, and the circuitry is quite easy to understand - a bank of capacitors is connected in parallel for charging with spark gaps that will cause the bank to be connected in series when fully charged - creating a brief high voltage pulse. This pulse will need to be further refined via the pulse forming network - the blumlein line seems to be the best option, as best I can tell. Essentially the blumlein line "tightens" the pulse, making it a much briefer duration and consequently more powerful. The final step will be the emitter. Most of the data on EM weapons I was able to find on the clearnet seem to place them in the microwave frequency range, though standard microwave antennas seem to be not frequently used. Instead a vircator seems to be the suggested option - basically it has a cathode and thin foil anode which, due to the intensity of the pulse, sheds electrons creating a "virtual" cathode that somehow transmits the pulse. Don't ask me to elaborate - I'm just a HAM aficionado, this is all black magic sorcery to me as well. The preferred variant of this seems to be the reditron (hey reddit is useful for once!), which replaces the foil anode with a thick piece of graphite with a slot cut into it. I'll attach the relevant papers I found on the topic. What turned me off from the idea (apart from the obvious legal concerns) are some videos I found from an Indian lab which had some rather impressive marx generators (albiet with piss-poor forming networks and emitters) that were only able to produce marginal effects on their test equipment (RIP oscilloscope). Link to follow.
>Test Video: https://www.youtube.com/watch?v=GRNQxxkjQag
>Channel in question (good videos of marx generators, highly recommend): https://www.youtube.com/channel/UCCNmmGnbPpWe-blXo1alumQ/videos