Getting started
The Setup Guide takes you through these steps the first time you open Bigshot. It takes about a minute.
Install
Open the disk image, drag Bigshot into Applications, and open it from there. The Setup Guide opens on its own and lists its steps down the side. Each one is ticked off when Bigshot sees it done.

Allow Screen Recording
Bigshot needs Screen Recording to take screenshots. Open System Settings from the guide and switch Bigshot on in the Screen Recording list under Privacy & Security. The guide notices the moment you do. If it keeps waiting, choose Quit & Reopen: macOS only gives the permission to a fresh copy of the app, and the guide picks up on the next step.

Pick your screenshot keys
Use ⇧⌘3 and ⇧⌘4, and Bigshot turns the macOS screenshot keys off for you; ⇧⌘5 stays with macOS. Or keep the macOS tool and use Bigshot's own keys, such as ⌃⌥A for an area. Press the key while the page is open and the guide shows that Bigshot heard it.

Take a screenshot
Press the key and drag over part of the screen, or click Capture an Area in the guide. The screenshot is saved to your Desktop, copied to the clipboard, and kept in the island. The guide shows it as soon as it lands.

Try the island
Bigshot hangs from the notch at the top of the screen, or from the middle of the menu bar on a display without one. Hover it to see your latest screenshots, hold the camera to choose a capture mode, and click it to open everything. The guide ticks each one off as you do it.

Find Bigshot later
Bigshot has no Dock icon and no window. It runs in the background, in the notch and in the menu bar. The last page of the guide dims the screen for a few seconds around both. If you cannot see the menu-bar icon, a full menu bar may be hiding it behind the notch, or it may be switched off in System Settings › Menu Bar. The island is still there.

Open the guide again
Choose Setup Guide… from Bigshot's menu-bar icon, or open Settings › General. Everything else is in the user guide, and fixes for common problems are in Troubleshooting.