Write A Riff
A riff is essentially just a melody. As usual, for melody writing lessons, check out MTM 17, 18 & 19for melody making basics.
Generally, when I think of a riff, I think of an intro riff like in MGMT's Kids or The Black Keys I Got Mine but a riff can also be used in place of (or alongside) the chords in a song, like in the verse of the Beatle's Daytripper, or as a riff/chords combo like in Seven Nation Army.
There are other uses for riffs, but try to focus on this type of riff for this exercise! Generally, a riff used for an intro or as a chord replacement has simple, memorable phrasing. If it gets too fancy or the phrases get too long, it might not have that anthemic effect. Also, this is a great opportunity to try a phrase scheme like ABAC or AABC or ABCC.
As usual, your riff can be an on-the-fly 8 second phone recording or part of a full fledged, multi-layered song recording in your DAW. Whatever you have time for. Whatever doesn't stress you out. The idea is to have fun!