I tried that method also a few years ago but could not get the consistency by eyeball and measuring.
I also tried using thin brown paper (from smaller bags of course) and running them through a laser printer to put some marker lines for folding.
That worked to an extent, but the grain would not always line up well in the final product.
Next, I just printed a brown shade on plain copier paper using a laser printer and ended up with what I thought was a reasonable result. The inside is white, but from the outside, they look pretty good.
I suppose it is also possible to print brown on the inside as well, but that seemed to be too much work at the time.
Here is the pattern that I uploaded a while back:
http://www.onesixthwarriors.com/forum/printables/216197-paper-shopping-bag-organic-grocery.html
I should have also posted a photograph of the result.
- Ivan.