There's a set of so-so directions on the Montour Trail website here.
My version: in McKeesport, cross the Yough via the bridge, then follow the main road along the Mon to the bridge in Glassport. Cross bridge and make left onto 837, then make right at Montour Trailhead a mile or so down the road. (My version doesn't follow the Steel Valley trail signs; those take you a bit more out of the way, but keep you off main road for an extra mile or so.)
I've done it a few times; it's pretty mellow, traffic-wise, at least on the weekend.