Mac software quick-fire questions

How can I run an unsigned app in Sierra?

The simplest way is to select the app in Finder, and use Finder’s Open command to open it. The first time you do this after downloading the app, you’ll have to opt to open the app despite its missing signature, but once it has been run, full Gatekeeper checks are not repeated.

How much free space do I need for High Sierra?

In addition to that required for a major macOS update, if you start up from an SSD, the installer will need extra free space to convert that to AFPS. That’s not readily predictable, but most who upgrade will find 10GB an absolute minimum, and 50GB more comfortable.