It’s really easy. And by creating your own stencils, you’ll save time and more time.
First, start a new Stencil. File ▸ New Resource ▸ New Stencil.
Draw your objects. If you solicit a lot of feedback, annotations get used a lot, so let’s just start there.
Then, save it!
Manual Guides aid with document layout. Not only do they give the eye a quick baseline for alignment, but they also affect Snapping behavior when aligning objects.
Quickly add a manual guide to your Graffle document by clicking and dragging from either the horizontal or vertical ruler. You can turn on the Ruler with Command-R.
Even more: You can also edit guides’ position in the Guides sidebar tab, copy them or move them to other canvases, and change their color.
Hiding layers in OmniGraffle is just one click away. To hide a layer immediately—so that you can work on another layer or, maybe, because it’s an old revision—toggle the 
(eye) icon.
New in OmniGraffle 7: choose a Keyboard Shortcut Set you might be more familiar with.
Choose Keyboard Shortcuts from the OmniGraffle menu bar. Switch from Default to Adobe or Sketch.
Of course, if you have muscle memory for individual actions or tools, you can change those as well.
By default, OmniGraffle opens the Single Pixel Grid when you starting a new document. Switch to something you prefer by opening Preferences, looking for New Documents under the General Preferences pane, and picking another template.
If each new document you create is a bit different, Open the Resource Browser every time you create a new document.
A stroke follows a specific line path and can be added to any line or shape. In OmniGraffle, a Freehand Stroke adds a bit more jaunt to a shape’s jive; it makes everything a bit more rugged. For when you want your mockup to look unfinished, beta, work-in-progress.
