SAVE YOUR ELECTRUM SEED. Write it down on a sheet of paper, in a text file and/or remember it. Just make sure that you still have access to it if you loose your Tails USB stick. Then you will always be able to recover all your bitcoins.
Do I have to do something in order to receive bitcoins? No, you just need to send the bitcoins to one of the addresses under the “Addresses” tab. It is not necessary to fill out the form under the “Receive” tab.
Use a new Bitcoin address for every transaction. You have many different ones to choose from under the “Addresses” tab and you should use them because it does not cost anything to use or create new addresses. It further strengthens your OpSec, so do not use one Bitcoin address twice.
Make sure you have enough bitcoins for your order and the shipping costs. A little extra left over is ok.