Careful planning and simplicity in design are key when it comes to baseboards. Not just in track layout requirements but also in the wiring beneath, especially in a modular design or one that is meant to be portable. If you can set out your track plan first then design and build the baseboards around it, there will be no reason to compromise the layout at a later time.
The foundations, so get it right!
Golden Rule 2 - Take care.
You cannot build a reliable well running layout on shoddy or poorly designed and constructed baseboards.
When you are happy and have bought the track, lay it out as per your diagram and check the plan works. If you lay it out on large sheets of paper (wallpaper is good) draw round it carefully then you can use this template to create your baseboards. If you are tight on space and are using small radius curves, even if you intend using flexi-track, it pays to buy a piece of each radius of setrack to use at this stage temporarily to check you are keeping things lined up. Doing this allows you to manufacture the baseboards so that strengthening battens are not just where your point motors need to go, or similar issues.
Consider when making the boards things like weight and portability if that is a requirement. Fixed boards for a home layout can be heavy as long as they are well supported, but a portable or sectional layout needs to be light but durable.
Thin ply with glued joints and only minor joint reinforcement can make for strong and light structures, almost like the construction of aircraft wings, with formers to make the landscape contours and skins for the track bed and land datums.
Plan how you are going to cut your chosen timber to maximise the usable areas and after all joints are dry, sand everything smooth and paint every surface to prevent moisture loss and any warping or splitting. A small amount of flexing is OK and will not be a problem.
You can use any paint, gloss for the edges for presentation and emulsion for the surfaces which also help to add a guide prior to adding foliage later. Take pride in making the carpentry presentable and looking professional, especially if it is ever likely to be seen in public.
Think about how you support the boards if portable. Trestles are easiest to make and also do not add weight to the actual boards and can be made almost infinitely adjustable for height and uneven floors.
Once the boards are made and the paint dry support them well and prepare to start laying track...now the fun begins!