Would you like step-by-step instructions on creating a bootable installer if you already have the official Sierra .app file?

If you need to perform a "clean install" or fix a Mac that won't boot, you can create a bootable USB drive (at least 8GB or 16GB required).

Finding a clean, trustworthy is harder than it should be, simply because Apple prefers users to upgrade. But with the legitimate methods outlined above—using Apple’s legacy support page, MDS, or verified archives—you can breathe new life into an older Mac without risking malware.

Use Disk Utility to erase your USB as "Mac OS Extended (Journaled)" with a "GUID Partition Map".