Go to www.onshape.com/features/custom-features and find a feature you want to use. You can also explore featurescripts.bubbleapps.io or forum.onshape.com for more user created features.
2. Clicking on the feature will bring you to an Onshape file explaining how the feature works. Simply click "+ Custom Features" to add it to your toolbar.
3. Once in your toollbar, you can use the custom feature like it was any other feature in your toolbar.