Start by reviewing Exploring SizeUp's AppleScript Features for info on how to run AppleScripts.
Follow these examples to get a feel for the Grids feature and let me know if you need help doing something specific.
Keep in mind, these grid actions still only operate on one window at a time, it basically just gives you the same control as Divvy.
Example 1
tell application "SizeUp" to fit to grid {0, 0, 3, 6}
By default, the grid size is assumed to be 6x6. The grid coordinates are 0 based, so this command is asking the window to start at the first grid from the left (0), and the first grid from the top (0) and resize to fill 3 grids wide and 6 grids tall, which is the same as SplitScreen Left.
Example 2
tell application "SizeUp" to fit to grid {3, 0, 3, 6}
This is the same as SplitScreen Right.
Example 3
tell application "SizeUp" to fit to grid {3, 0, 6, 12} grid size {12, 12}
For more control, you can define the "grid size." This allows you to position a window 50% wide (6 grids of 12) in the center of the screen.
Example 4
tell application "SizeUp" to fit to grid {0, 0, 3, 6} on monitor 2
If you have multiple monitors, you can even specific which monitor you want the window to be resized on. Monitor index is 1 based, so this example puts the window on the 2nd monitor.