Choose File ▸ New ▸ Document… (Ctrl+N). Set the page size, orientation, number of pages, facing pages, columns, margins and bleed, then click OK. To reuse a setup, give it a name and save it as a document preset in the same dialog; File ▸ Document Presets ▸ Define… manages the presets.
Master Page reads and writes IDML, the open interchange format that professional layout applications export. Choose File ▸ Open… (Ctrl+O) and pick the .idml file, double-click it in File Explorer, or pick it again from File ▸ Open Recent. Fonts this PC does not have are drawn with a stand-in and highlighted (see Preferences ▸ General).
A native .indd document is converted with the import command of the File menu, below Convert PDF to Book…; the result opens as a new IDML document.
File ▸ Save (Ctrl+S) writes the IDML package. Only the parts you changed are rewritten, so a file you did not edit stays byte for byte the same. Use Save As… (Ctrl+Shift+S) for a new location and Save a Copy… (Ctrl+Alt+S) to keep working on the current file. File ▸ Close (Ctrl+W) closes the document.
File ▸ Document Setup… (Ctrl+Alt+P) changes page size, number of pages, facing pages, bleed and slug of an open document. File ▸ File Info… (Ctrl+Alt+Shift+I) shows and edits the document's title, author, keywords and other metadata.
After an unexpected quit, Master Page offers to reopen the documents that had unsaved changes as untitled “(Recovered)” copies. The offer can be turned off in Edit ▸ Preferences… ▸ File Handling.