Bit of both, but most often the belt pops into place, as the belt teeth fit into the toothed can wheels. There's also often an element of fitting the belt so it's off the end of them cam wheel to allow the idler to be fitted, as there isn't any sideways give in the belt. So you have to slide the belt on/away from you towards the engine in order to get everything in place, before you can tension the idler.
Sorry that not what you asked

being specific, from crank to cam 4, you have the idler in between. You can run the belt up over the idler, then round the cam wheel, pull it tight and it might very well pop in as the belt teeth drop in to the cam wheel teeth. Or, you could remove the idler, run the belt loosely somewhere thereabouts to cam 4 and then fit the idler and see where you are. Might need to shuffle the belt back or forward a tooth to get the idler in loosely. Then just lean the idler against the belt to take up the slack enough for the belt to look like its on a straight run, then nip it up.
There's no absolute one way as to how to do it, as long as the marks line up after its cranked over a revolution by hand, the tensioner is on the correct mark, and all the bolts are torqued correctly, you'll be fine.
