1.First id be sure to draw in layers. In big images id first draw it on the left side in 2d first id only draw lines just 2d lines
2. save the image then Id crop a small section of the image by drawing a red square Be sure the red square is a layer you might want to change the compoiitng method to difference. over your cropping area turn the delta down to zero crop that square! “save as” the image then undo now when you open your image double it on the x axis so you can draw in 3d.
3.At this point you are still only drawing lines but the main difference in this image is that you are now drawing in 3d. Now after you do this you start filling in your image the dead giveaway to see if your on the same “delta level” is as you draw look at the right side of the image if the pixel your drawing seems to fit into place relatively but not exact as your left side then you know your in the right place if the pixel is just there and does not corospond to the left image change the delta till it does. This is the way to flesh out the whole image.
4. As soon as you are done with one part put it in with layers like a jigsaw puzzle fitting in where it should go according to the outline. Do this and eventually you will finish the whole big image.