Summary: Click the Estimates tab in your project and click the blue plus button in the bottom right corner. You can name the estimate and apply a template at this time.
To build a line item estimate, click the estimate to open it and then add items by clicking the blue plus button, selecting the desired items, and clicking "Done" to save.
To build from a template, apply a template when creating the estimate, then click the estimate to open and modify as necessary.
Watch the video below to see the process:
Note: You can save any line item estimate you create as a template to apply to later projects. Click the 3 dots to the right of the "Override" switch in the top right corner of your screen when building the estimate and select "Save As Template".