Easy gridding

 I have read a few articles about the grid method that involve drawing a grid directly on a picture or on an acetate sheet to be lain on a photo. Then drawing a grid in light pencil on the drawing paper to be erased as you work.

To make a cleaner process I use my computer to help with the grids.

First take a downloaded or scanned image and load it into GIMP. (Though you can probably do something similar with photoshop, but GIMP is free)

Then select 'Image>Canvas size...' and adjust the canvas to the size of your drawing paper. You will probably also want to change the aspect ratio, by hitting the small chain symbol next to the sizes. I have selected the canvas measurements in millimetres here and the closest I can get to A4. (I normally work in A2 but A4 here should work for most printers) 

Hit resize and you are back to the image window. Then right click on the later in the toolbox and select 'Scale layer...', or in the window select 'Layer>Scale layer...' and adjust the layer to fit in the canvas as you want, you will also probably want to move the later round till it is how you want it to look on the page.

Next create a new transparent layer with 'Layer>New Layer...' 

Now to add the grids. First select the foreground color in the toolbox and change it to black.

The with the new layer selected go to 'Filters>Render>Pattern>Grid...'. In the new dialogue set the cell spacing that you want. Here I have used a spacing measure of millimetres again with a 10mm cell and I have used an offset of 0 in both directions that will help later.

At this stage you will have an image with a 10mm grid overlay (or whatever size grid you used). 

Now when I am working on a grid with a lot of cells, I can get a bit lost so I also like to use a reference grid. So again choose 'Layer>New Layer...' and create another transparent layer. Then with the new layer selected go to 'Filters>Render>Pattern>Grid...' again. But this time select a larger grid spacing and a heavier line. Here I want to mark ever 10 cells so I have used a grid spacing of 100mm minus 1mm for the extra thickness of the line.

Now the image has a light grid and a heavy reference grid. 

Next in the toolbox turn of the underlying background picture by toggling off the small eye by the image layer, and print out the grid...

Now re-enable the background image and save the file. Which you can now use as the reference image with grid.

Next I stick a sheet of drawing paper over the reference grid that I printed out

 and place them both on my perspex drawing board.

Then placing a light behind the perspex I can use the grid reference without drawing lines on my paper, and also can zoom into the reference on the computer with the grid lines in place.