High blip and welcome.

Electricity is an old topic at the library you might want to give it a search. There is an in depth one here

Basically you have a good understanding of the biology. Myt from there engineering side there are bigh risks associated with DIY electrical devices. Without going over too much old ground my advise as an electrical engineer is dont bother the risk is too great. Besides the comercial units starting arround $125 US are pretty cheep. You could spend that on parts alone and still not have a reasonably safe toy.

Probably the main reason you wont find too much on spacifics on how to build one is liability.

If you just have to build one there are published standards for these types of things. You might try the websites for UL and IEEE for starters.

Have a shocking good time but a safe one. Cus you want to do it again.