Follow these four easy steps to convert your Sketch layers to grayscale.
Insert a rectangle spanning the entire content (all layers) you want in grayscale.
Select the rectangle. In the Inspector pane under the Fills category, change the Fill color to white.
Useful Tutorials — Branding and Logo Development
This is a useful tutorial on how to import an image from your sketchbook into Sketch, working with the vector tool, to develop your logo or other graphics: