first, you deconstruct the pixel into its R, G, and B

if in 565, divide the G component by two, so that everything is from 0 to 31

grayscaling is done simply by weighting the colors by percentages.

a good weight to use is:

Blue: 11%
Green: 59%
Red: 30%

so, now calculate:

NewRGB=(b*11+g*59+r*30)/100;

use this new value as the new red, green, and blue components (if in 565, multiply by 2 for green)

now reconstruct the pixel from these RGB values, and put the pixel

if you want a "yellowed paper" or "antique" look, multiply the blue by 7/8 or 3/4 (or whatever fraction you like, based on your preference)

you can also get a nice "smudge" or blurring effect by taking some areas and apply an anti-aliasing algorithm between every other pixel.

(if you want "direct" memory access to the surface)
Lock()
Unlock()

(or, if you want the slower GDI route)
GetDC()
ReleaseDC()