Credit to codmoddd1234 who sent me the bulk of this stuff, I adapted it into a tutorial with his permission.
Step 1
Right click in Radiant, go to script > vehicle. Give it the following key-value pairs: You can use any xmodel. The rubber_raft vehicletype allegedly makes better curves, according to codmoddd.
Step 2
Right click in Radiant, go to info > vehicle > node_rotate. The node_rotate means the vehicle will follow a curve; if you want straight lines, select node instead of node_rotate, you can also use combinations of both. Deselect everything. Select the vehicle, then the node_rotate, then press W to connect them.
Give it the following key-value pairs: Play around with the speed and lookahead settings later as you wish.
Step 3
Place another node_rotate or node (or copy previous one, but if you copy it remember to remove spawnflags KVP from new nodes). Deselect everything, select the first node, then the new node, press W to connect.
Repeat this step until you have the path you want. As you place new nodes, you will see a red curve appear between the first and last nodes (select any node to see it). This is the path the vehicle will follow.