GIMP | Tapering rivers with RobA's script

[Weltall_BR]

So I'm trying to draw a map based on Gidde's guide. I'm really bad at hand drawing and it has been really challenging to draw it, but I manage past sketching the mountains and hills (and also the plateaus, which won't be on the final map but will help me with setting climates, define borders and stuff like that).

I got stuck with drawing the rivers. Gidde advises to use RobA's script to draw rivers, and that is what I am trying to do. Somehow, I just can't do it. I'm clearly doing something wrong, but my lack of familiarity with GIMP and my poor drawing skills won't let me figure out what I'm doing.

As you can see in the picture below, I draw the path. But when I try to taper the river, the script generates it in a completely different place. Here is the print screen of what I am saying (the tapered river is the thin black line, of course):

f9592575157_10ac1aa5af.jpg

Can someone please tell me what I'm doing wrong? RobA's guide says that I must draw the path and them change the tool to paintbrush, but when I do the path goes away. Sorry if this is a really noob question, but I have tried hard to figure out what I'm doing wrong and simply couldn't!

Thanks in advance!