1. Copy to clipboard the url of the article you want to link to.

2. In the article that will contain the link, highlight a word or phrase (choose the anchor carefully--don't link to something that the reader might not expect from the anchor itself).

3. When you highlight a word or phrase in Medium, a toolbar will pop up with a green chain link symbol (among other options). Click the link icon and then paste into the box that shows up the link from the clipboard.

4. After you publish your article, test the link by clicking on it.