In the operations (PART/OPS) tab, you specify each of the repair operations you are billing for in your RO. There are several built in shortcuts that can make this quick and easy.
In general, you "build" a line by first entering something for Operation by typing or choosing from the drop list (which you can edit). Then, as an option, click or type a Position. (Left, Right Rear etc.) Specify a quantity if necessary.

In the example below. The section for Tires has been clicked on the right (circled in blue). Clicking the green bar at the bottom of the section list opens to a choice of pre-set jobs that you define. Just click to select. This will bring in price, labor time etc from you custom job lists in setup. When you enter a cost, you can mark it up by any percentage you want.

Use the button New Part to add another line item part to the RO. A line at the top of the tab tells you which part you are working on; in this example it's part 1 of 1 parts.
Use the buttons First, Previous, Next and Last to move from one to the next.