The method stokbrokin laid out works well and I did the same thing. But I also bought the way you mentioned with Coinbase and BitMart, since I already had a Coinbase account. I am guessing you do too since you mentioned them.
My last buy was easy IMO. I bought ETH in Coinbase and transferred it to BitMart. Once in BitMart, trade ETH to USDT. Once you have the USDT you can buy SafeMoon.
Once those account are open, I found that easier.
Coinbase was linked to my bank, so funding takes minutes as do the buys and swaps.