I always use the grid method for my sketches before I start painting as it helps to ensure all the elements are in the right spot on the panel. For example if I'm using a 12”x 12” panel I will usually apply the grid with 12 squares across the top and 12 squares down the side therefore creating inch size squares.
The wonderful thing about using a grid is you can also use it for scaling up and down to any size that you want. If your painting has a lot of intricate details, it may help to divide the grid into more squares so they are smaller, for example on a panel measuring 12" x 12" divide the panel into a 24 x 24 grid, this will assist when trying to include finer details.
First, apply the grid to the reference image, then using a graphite pencil divide the panel or canvas you are painting on into square inches. Follow the image diagram and copy the contents of each square into the squares on the panel.
Accuracy isn't as important when you're working on a landscape painting because every element doesn't need to be precisely in the correct position. We just need to get elements into roughly the right places, for example, we will want the horizon line to be correct, but the surrounding areas can be adjusted slightly, or can be as accurate as you like.
However, it is essential when working on a portrait or figure, to get the sketch drawing as accurate as possible, with everything in the right place and the right size. The most important aspect of getting the portrait or figure looking right, is ensuring the proportion is as close as possible, using the grid makes this a lot easier.
Remember, it’s best to remove the pencil grid lines from the lighter areas before you start painting to prevent it possibly showing through your paint, otherwise you may need more coats of paint to cover it. It's important to eliminate the unnecessary use of extra paint.