Unless new information is released that obviously changes my perception (or makes it clear that this scenario is wrong), this will be my choice for most likely actions leading to the deaths. Not that I don't think there could be other possibilities that I haven't thought of, but mainly because I hate typing and my opinions don't really change anybody else's so I'd rather just spend my time here reading others' thoughts and occasionally commenting on them.
I see BG pulling into the cemetery and parking his car where it's not obvious from any/most directions. He takes out a backpack/duffel bag that is holding an extra set of clothes, any supplies needed for the abduction (zip ties, duct tape, or whatever to bind and silence them, any larger weapons/tools he wants to use, possibly even a shaving kit, etc.), and possibly food and water in case he's waiting for a long time. From the cemetery he goes as discretely as possible to the beginning of the trail so that anyone who might see him entering the area will believe he came from a different direction. He then follows the trail to the bridge, and then he takes the trail on the north side of the creek to the place where the bodies were found. He leaves the pack/bag there and follows the path back to the bridge/trail.
Once that is done, he finds a place to wait for a likely victim to come by. He needs a place where he can remain somewhat hidden, but not so far away the girl/s would be gone before he could get to them. Mixed in with the brush and trees, the slope would be a reasonable place because most people would be unlikely to be looking for anyone there, and he wouldn't be noticeable unless someone was actually looking right at him. Without him realizing it, the girls may have been close enough to see him heading down there to hide, which might seem pretty odd to them.
When he sees them crossing the bridge, he decides it's a great opportunity because they appear to be alone and distracted with their pictures and videos. Because it takes a while to get back up the slope and over to the bridge, there's no sign of him in the picture of Abby. The girls see him coming out of the wooded area and think it's rather strange, so they keep an eye on him. When he starts getting too close, they start heading back to the other side to try getting to their pickup spot or find someone else who feels safe. As they approach the bridge, Abby goes first and Libby starts a video of her and BG. The part we've been shown is actually the end of the video part. They walk to the first platform and wait for him to pass by, and then head across. He goes past them and turns around as soon as they start moving.
When the girls get to the end of the bridge, he's right behind them and already has a gun out. He forces them to go along the path to where they were found, murders them, shaves, changes his clothes and heads back to his car at the cemetery. It's possible that he used the outbuilding across the road to park his car instead, but I think that might leave him out in the open for too long.
A lot of details are missing, but that's enough to get my point across. MOO