Hello !
Personally, here's how I do it:
1 - Use a dev texture, vary the textures for the floor, walls, ceilings.
2 - Place the blocks that will determine the different paths. Use a grid of 16 is perfect for the beginning.
3 - Place the rails and determine the path of the cart.
4 - Cut the heights I advise you to read this tutorial
http://forums.tf2maps.net/showthread.php?t=12605 .
5 - Close the map.
6 - Place a basic lighting everywhere.
7 - Place areaportals hints etc..
8 - Test.
9 - Placing entities and models related to the gameplay (doors, cart, life and munitions).
10 - Retest on a server with players.
11 - Modify the map according to feedback.
12 - Texturing.
13 - Creating the lighting
14 - Create the displacements.
15 - Props
I can only wish you good luck
