The technique I use to draw is called blind continuous line drawing, which is where you only look at the object your are try to draw and only glance at your drawing.
You also can't take your pen or pencil off the paper which creates all the extra lines and by not looking at what your drawing you can really capture the form of the object especially if you draw very slowly and carefully trying to include and notice as much detail as you can from your subject.
Normaly I add pastels for colour but sometimes I use use water colours or pencils which create different effects, but I prefer pastels because of the bolder colours and better shading ability.
I also only drew in ink pen because of better quality of lines that can be seen a lot clearer.
To create the different variations of the same I image change them on my phone using the built in edit option.
Normally I experiment with the contrast, hue and saturation, along with adding effects such as gothic noir, posterize and negative etc to see how this will change the image and what I can create with them.
Hope this helped if you have any more questions feel free to ask and thanks for your interest ☺