Planning for positioning straight track is pretty simple; just put a ruler or straightedge down in the desired position, and draw a line. Curves, on the other hand, require some planning. Curves need both a minimum radius and an easement. The easement is the slow transition from straight track to the desired radius, and then back into another easement out to the next straight track. If you are an artist, you might be able to draw a gradually-tightening easement, but I prefer to use a simple tool. The photos are of my 2015-2021 layout, where the track was on a continuous curve. Its length was 8 feet. Since I didn't have anything flexible that was that long, I decided to make a temporary jig. I had a collection of 1/8" square wooden strips that bend nicely. I put them together in an offset, alternating pattern until I reached the full length. To do this, I placed a metal weight on the first strip, and used small lightweight plastic clamps to hold the strips together.
Most of the time, the lines I draw on the layout surface are for where the outside edges of where the ties are to be positioned. I don't find having a centerline drawn to be of much use, unless you are doing high-level rough planning. Having a line against which to lay the ties is much easier. This photo shows the temporary jig put in position along the desired curve. A wooden board in the bottom-center of the photo is used to make sure that the first foot or so of this track is perfectly straight. That end of the jig is clamped to this board with two metal clamps. By gradually teasing the jig over, I wind up with a natural easement in the track as flexible material will naturally tend to have an easement in them when bent. Metal weights hold the jig in position. It is then just a simple matter of tracing a line along the outside edge of the jig.
For parallel curve track, I use my track separation gauge. It has two slots in it to allow it to be positioned on two rails, but in this early stage, there are no rails. I simply line one slot up with the outside edge of the previously-drawn edge of the ties, and position the long flexible jig such that it lines up with the other slot in the gauge. When using a gauge such as this, be sure to match apples to apples, i.e. inside curve tie edge to inside curve tie edge.