Start of by getting the Solidity code for an ERC20 token from this GitHub gist.
Enter the URL for the Remix Ethereum IDE in your browser.
Under "Environments" click "Solidity".
Under "File" choose to import from "Gist".
In the pop-up that appears enter the ID, a779c0277f2fc5c935e2b2f62e49da49, of the gist you just viewed.
If the import was successful you should be able to open the imported code from the file explorer on the left hand side menu.