There are two levels of getting the tracks in the right place, first of all there is the 'is the track in the right place' issue, and then there is the 'is it perfectly straight' issue.
To get the track to line up I often make use of a laser line. This tool provides a perfectly straight line right across the room allowing the angle of track to be set correctly so that the other end ends up in the right place.
The center studs are very useful as they show the laser beam very well. Here I am getting the overall alignment of the track and bridge correct.
Once I had the straight section above planned, I needed to make a 3m diameter 180° turn, and I wondered how I would get the track in the right place on the curve. The solution was to temporarily attach a hook to the ceiling joists to which I attached a nylon rope.
Using some trial and error I was able to locate the correct position of the eye (i.e. in the center of the curve). Once I had it right it was easy to draw the curve for both the outer edge of the ballast and the center of the track. I started at the edge of where the track had to end up and adjusted the rope to be the correct length.
It is important that you use rope that cannot stretch.
Now to get track perfectly straight, wherever possible I use the straight Merkur ballast which holds the flex tracks perfectly straight, but when it came time to lay track across a bridge (where there is no ballast) I realized I needed another solution.
I got some scrap ¼" plywood and cut a strip 16mm wide, just the right size to fit between the running rails. By pressing this into the track I can ensure that the track is straight (in the horizontal axis at least)
In the picture above one can also see that the bridge supports have not yet been adjusted to ensure that there is no dip or hump going over the bridge.