Note: I'm using the development version of the GIMP (version 2.3.18), so the location for the guides tool may not be the same.
I've made a quickie template just for this tutorial. This tutorial will be quick, straight-forward and simple.
After making a template for yourself, select the rectangular marquee tool. Make a selection on a section of your template like so:
FIgure 1
Then go to Image > Guides > New Guide from Selection.
Repeat the process until you have filled the template with guides:
Figure 2
Now you can select an area with ease:
Figure 3
And Edit > Copy Visible, then File > New, then Edit > Paste (you may have to make a new layer for the float) and save as an image:
















