While all of the sketching is done by hand, I initially copy the outlines from photographs. For me it's more of a fun way to get realistic proportion and really capture movement in a picture, than it would be to sketch freeform.
On top of this 2B outline I then tend to shade what seem to be the darkest areas on the photograph with a 4 and 8B pencil, so I know the other areas can't be allowed to gain more tone than these.
Next I outline the key elements that are in front of or above others, such as people and waves in most of my work. I use a charcoal pencil. It gives a good strong outline that I can blend in or out from, but I discovered it's important to work from the top left or my hand smudges it as I go.
Lastly I add shape and form. I use a blending stick for skin, copying the shadows on the photographs. I low/highlight with either my 2B or a white pencil, and blend or add detail with the 4B or the charcoal.
The final stage I absolutely love and it came about mainly from the need to print to avoid the charcoal blurring. I screenshot the images on my phone and play around with filters on them through instagram. I used to feel guilty for the odd lightening or sharpening but as a wise friend said - it's all part of the creative process! The originals are sprayed with a fixer and taped into a sketch book and the digital images are emailed for printing. It's usually having taken the photo at night in the lounge that gives the sepia tone than any major instagram filter.