Glyphs automatically generates most of the common features, click the Update button in the lower left to initiate that. The feature generator is based on certain name conventions and suffixes. E.g. all letters with names ending in ‘.sc’ are being located in the small caps feature. Letters described with more than one letter name joined by underscores are being recognized as ligatures. Find a complete list of the suffixes here:
Glyph naming scheme for automatic feature generation
To write features manually, uncheck the ‘automatic’ checkbox or create a new feature by clicking the plus button next to the "Features" entry in the sidebar. Edit the name of the feature (or class) directly in the list. Glyphs uses Adobe FDK syntax. Find details at Adobe Opentype Feature File Syntax.
Put all information that needs to stand outside a feature in the prefix. Glyphs will put language systems here.
The bottom right area is for notes.