How can I quickly set up a garter stitch chart?

There isn't currently a one-click option in Stitchmastery to create a garter stitch background, but this can be quickly achieve by using the select tools and the Fill tool. 

Select all of the wrong side rows using the Toolbar Select menu option Select Alternate Rows starting at First or Select Alternate Rows starting at Second as appropriate. The Select menu icon can be found on the Toolbar just to the left of the Redo icon. If you hover the mouse over it the word 'Select' is displayed.

Once all the wrong side rows are selected use the Fill mode to fill them all with Purl stitches. To do this, first activate the Fill mode by clicking on the Fill icon on the toolbar. The Fill icon is a small paint tin towards the right hand end of the toolbar. When deactivated a red line is displayed across the paint tin. This red line disappears when Fill mode is activated.

Once Fill mode is activated click on the purl stitch in the stitch palette. All of the selected rows are filled with purl stitches.

Be sure to deactivate Fill mode as soon as you are finished using it. It is a powerful tool but it can surprise you if left switched on all the time.