There are two ways to use a smoke grenade, either to bring units into an advantageous position for a fight or to directly bring units into your enemies, the former is easier. At the instant the smoke grenade detonate, there is almost no effect on the LOS, wait a few seconds for the cloud to fully form and block off the unit's vision before moving out from your current cover. It only obstructs line of sight(LOS), if your units catch enough attention inside the smoke(shooting, running near to the edge of the smoke screen) they'll still be spotted and they'll be as good as not in cover.A couple of things to note about the smoke grenade