Get them (Engineer inf, or engineer/mine plow AFV) adjacent (can also be in the same hex, if they've stumbled into a minefield for example) and facing the obstacle. Then, just have them sit tight, and they will do their job. Hopefully.
Regular grunts have to be in the same hex, and will take longer to clear the obstacle.
Suppression will slow their progress as well, so they don't mind second hand smoke.
