How to make a line draw on in after effects

It has been a while since I did this in the first section of D&R so I forgot how to do it!  Apparently Trim Paths is the way to go.

Basically create a path using a pen (I was tracing a river to make it stand out), disable the "fill" (in the same tool bar as the pen tool), and change the stroke to the colour you want for the line (and thickness). 

If you are doing camera moves on the map, make sure you set the shape layer to a 3d layer before you get carried away making a good line.

Then Select the layer and add a trim path parameter (use the start and end to animate the sequence):

If none of that made any sense (it is actually just to jog my memory cause I forget this stuff all the time!), watch the video here:

Pretty good tutorial here: (go to 9:36 for the useful bit):

That is all.