Skeleton's base grid is a variation of the 960 grid system with 16 columns. Each column has 40px. The syntax is simple and it's effective cross browser.
It would be very wise to use Firebug (in Firefox) or Developer Tools (in Chrome), it will surely make your life easier.
One + Fifteen:
[grid1 first] one [/grid1] [grid15 last] fifteen [/grid15]
Two + Foureen:
Three + Thirteen:
Four + Twelve:
Six + Three + Three + Four:
[grid6 first] six [/grid6] [grid3] three [/grid3] [grid3] three [/grid3] [grid4 last] four [/grid4]
One-third + Two Thrids:
Please look at this article in the Article manager to see how the shortcodes are added.