How can I quickly and easily pencil in guide and slant lines for lettering/calligraphy?
Note: This is a more expensive, albeit an efficient method.

Requirements: 


A bright lightbox, or lightpad
A grid printout


An alternative would be to get a lightbox/lightpad, and place gridded paper beneath the sheet you are drawing to. When illuminated, the grid should be visible through your worksheet (this depends on the brightness of your lightpad, and the thickness of the paper you are writing on). 

I have tested my own lightpad with 300gsm watercolour paper and it shines through quite well (it's a Huoin, though there are similar products available on Amazon. Make sure your lightpad is bright enough to shine through your desired paper! My lightpad's brightness is advertised as "1758 cd/ m2, luminous flux: 1800~2100 mcd".)

The grid that you use will depend on the size of the letters and your choice of calligraphy style. A quick google image search for "calligraphy grid" yields plenty of useful grids, including italicized grid lines for italic scripts. A simple printout of the grid is all you need, printed at the scale you like.

This method allows you to work at a faster pace, and reuse the paper grids again and again.