Controls
The action-keys, which can be changed in Options - Controls, are defined in two sets:

Myself - For the vehicle the player is driving:

RIGHT CTRL + F = Start/stop following the vehicle that is in front (if possible).
RIGHT CTRL + W/S = Decrease/increase following distance.
RIGHT CTRL + A/D = Adjust left/right offset when following.

Behind - For the vehicle that is following behind me (if any):

RIGHT SHIFT + F = Stop the follower.
RIGHT SHIFT + W/S = Decrease/increase the follower's distance to me.
RIGHT SHIFT + A/D = Adjust the follower's left/right offset.

Note: Each set of action-keys must use the same modifier-key!

Switching it on/off
Do please note that, when loading a new map or savegame, the vehicles have not yet placed enough "trail crumbs" for other vehicles to follow. So do remember to drive the leading vehicle some distance, before setting up the follower vehicle.

To follow some vehicle, point your own vehicle towards it and press the start action (RIGHT CTRL + F).

If no "trail crumbs" can be found, or the vehicle already is followed by another, a warning will appear and you will have to either move a little bit further towards/back, or change to follow another vehicle.

To stop following, press the same action again (RIGHT CTRL + F).

Distance and offset
The follow distance can be set using RIGHT CTRL + W/S. Do please note, that the distance is not measured in meters, but in "invisible trail crumbs" units. Also note that vehicles that have no traffic-collision-triggerboxes can not be set closer than 2 distance units.

Left/right offset is set using RIGHT CTRL + A/D. - HOWEVER! The offset is calculated from the trail that the first vehicle in the convoy, i.e. the leader, have driven.

Credits:
Script: Decker, zVoxty