If you do not have Apple Id, create one at developer.apple.com. Then, if you are a part of a team, someone needs to add you to the team at developer.apple.com.
Install Xcode from App Store.
Open project (topee/Topee.xcworkspace).
On the left, select Demo. Then, in the center, select General. Set your Team in the Signing section.
Command + Shift + K cleans the project (may be needed if you make changes in JavaScript). Command + B builds the project. Select Appex > My Mac on the very top of your Xcode window. Command + R runs the project. Select Safari. Once Safari is running, go to Preferences, then Extensions and check your extension.