With any track, you want to support it well.
Make sure your train table or benchwork has a firm, even surface. If you have changes in elevation, make sure they transition to level smoothly. If you have seems in the table top, make sure they are completely level and well supported.
Map your track centerline carefully along the surface. A yardstick works well for straight sections. Curves can be mapped by making a tramel from a yardstick or string attached to a point at the center of the circle.
Some sectional track has plastic roadbed already attached. Obviously, that can't be done with flex track, but there is a good alternative. Several companies make roadbed from cork and foam. Roadbed should provide a smooth but firm cushion for the track.
Split the material down the center and then lay the halves along the centerline of the track. You can buy pre-shaped pads for switches, or simply cut and splice the roadbed.
Tip: Profile rough cork edges with a rasp and fill voids with drywall putty before laying track to make ballasting easier later.