Hang the scrim properly. Hang the scrim not only from the top, like a regular theatrical curtain, but it should be weighted on the bottom as well to avoid unwanted wrinkles and folds. Bottom weighting is especially useful if the scrim is being flown in or out of the fly loft, since it will prevent the flowing nature of the curtain.
Light the area downstage (in front) of the scrim, keeping all lights upstage of (behind) the scrim turned off. From the house seats, the scrim will appear to be opaque and will look like an ordinary drop curtain. The audience will see only the action in front of the scrim.
Light the area upstage of the scrim, with the downstage lights turned off or at a lower intensity than those upstage. The audience members will then be able to see through the translucent scrim to all the action and scenery hidden behind it. Note that they will still be able to see the action downstage as well.
Experiment with different ways of lighting the scrim. Many times, the way a scrim is lit is up to the individual lighting designer, based upon personal experimentation and preference.