For this achievement as well as "Team Work" - you need to play in co-driver mode which can be selected from the main menu by scrolling down to 'Multiplayer' and along to 'Co-Driver'
It'll then allow you to search for a quick game, browse lobbies or create a lobby. I personally did this by creating a private lobby and boosting this with a 2nd Xbox
When setting up a lobby, you can select your role so for this achievement, you'll need to select 'Co-Driver' but all other options i would leave as default - except Damage which is easiest to have on 'Visual Only'
Once in the lobby, invite your friend / boosting partner / 2nd account and also select your preferred course. I did this on the short 'Wales Shakedown' which is just under 2km in distance and can be done in around 2 minutes (you'll find Wales under the 'Extras' option when selecting your rally location)
With both places taken in the lobby you can ready up by pressing
and race will begin. As co-driver, you don't really need to press anything as long as the person driving navigates the course but if doing this legit or with a boosting partner, it may be best to press the button prompts along the way
Once the event is finished, your achievement will pop. No need to set a winning time or anything as the achievement is just for completing the race