you should have your bitcoin cash saved in a private wallet, in case that is not so you can exchange your bitcoin to bitcoin cash in an exchange like bittrex. you ask for the receipt address on the bitmain website and you make the transaction from your private wallet or exchange.
Thank you ldah94 but I guess what I'm asking is that has anyone made a transaction via Bitcash to Bitmain for the purchase of buying one of Bitmain's machines? if so (or not but knows the transaction process) than please let me know. I'm worried as there are fake wallets sites and exchanges out there and since I'm a newbie I'm worried that I'll be scam. For example, use exchange A and than Wallet B to send cash to Bitmain etc. Please let me know the names of these websites (assuming that they are legit and you used them before) as a starting point. I'm assuming that most ppl have gone through this process / transactions before since there are a bunch of sellers selling these machines x 2-3 times the price they must have gotten from Bitmain at one point. I'm not looking to buy and resell but rather to buy and put these machines to work for myself. Thanks!