Grab the top of the ear you are putting the earbud in with your opposite hand and gently pull up and back towards the rear of your head.
Grab the earbud with the other hand, forefinger and thumb, and slide the earbud into the ear canal. To help facilitate this, some back and forth rotation of the earbud while it is being inserted can help. It may also help to open your mouth, which opens your ear canal for easier insertion.
The earbud should have a snug and comfortable fit, but should not sound like you are underwater! If you feel like you are swimming, pull the earbud out a little while still maintaining fit in the ear.