Once you've logged into Snapshot, the first thing you'll want to do is take a snapshot. Whether you are working within a specific org or you are working on between org actions like migrations or comparisons, the snapshot will make it all possible. To take a snapshot, you'll want to drag a blue snapshot icon from the 'Tool Pallet' in the bottom-left.
NOTE: When first using Snapshot, you might drag on the blue snapshot icon onto the workspace and immediately are brought to the Full Snapshot interface. If you do not want this behavior and instead want the choice to right-click for both options, go the the Snapshot settings in the top-left. Choose Snapshot Preferences from the menu, click on the Extras tab, then in the top right, choose 'Don't Automatically Open the Default Dialog' option.
You can choose to take a Full Snapshot, which captures all of the metadata within your org, or you can take a Partial Snapshot, which allows you to pick and choose which pieces of metadata you want to capture. To learn more about Full and Partial Snapshots you can click the links below: