I have been doodling for as long as I can remember. It got me in trouble when I was in grade school as I used to use up my tablets with doodles instead of with my practice of printing and arithmetic.
In class lectures in high school and college I had just as many doodles in the margins as I had notes. Fortunately, I could look at what I had drawn and remember what the teacher had been talking about.
Later, sermon notes included many little designs that later became cards or scrapbook page sketches.
I am not one to carry a sketchbook along with me (just one more thing for an already heavy purse) so I don't sketch much 'on the fly'.
I do have a sketch pad that I can use when I intentially sit down to do some doodles, though.
I use an eversharp number 2 pencil because I can't stand the way regular pencils get dull so fast. I use a Mars eraser as it is easy on the paper surface.
When I have a doodle or sketch I like I use permanent marker to ink it and then erase the pencil lines.
These get clipped out and filed for future reference.
After refining further, they might be turned into a sketch for a card, a rubber stamp to carve, or a watercolor design.