Please note the process of opening your account, getting it verified, then transfering funds to it takes about 4 or 5 business days in total therefore please do not lose too much time in starting the process if you are interested in buying one of the sweatshirts.
Step 1 - create an account
The whole process is very well explained here.
Step 2 - Add funds to your account
There are several options available.
Go to https://pro.coinbase.com/trade. In the “select market” space, type ETH. The different trading pairs will appear. If you have added USD to your account, please click on the ETH-USD pair, otherwise ETH-EUR pair.
Once this is done, you will need to click on the BUY button, but first a few things. By default, it will be a market order and this is fine in 99.999% of the cases so we recommend you not to worry too much about it and leave it as is. For the amount, you can click on max just below if you want to convert all your EUR or USD to ETH. Otherwise put the exact number of EUR or USD you want to exchange with ETH. Then you can click on BUY and confirm.
That’s it, then go to “Portfolios” at the top right corner and you will see you have Ethers.
Now, it is time to create a metamask wallet, so please follow the rest of the instructions here.