You can use the Shift and Trace tool by selecting it from the Tools toolbar or from the Shift and Trace toolbar. When selected, Shift and Trace becomes automatically enabled.
Likewise, when selecting the Shift and Trace tool, Onion Skin becomes automatically enabled. It is through Onion Skin that you will see the outlines of neighbouring drawings and hence be able to trace over them in the current drawing.
Once enabled, you can choose the amount of previous and next drawings to include in the Onion Skin as well as the options to use the Shift and Trace tool with.
NOTE If you need to use the Shift and Trace tool often, you can add the Shift and Trace toolbar to your workspace, which will allow you to quickly select and configure it—see Shift and Trace Toolbar.