Extract the zip into your x64dbg folder, the one that contains
release. The plugins land in release\x64\plugins and
release\x32\plugins.
The plugin binaries are unsigned. x64dbg loads them without
complaint, but antivirus software may flag a debugger plugin that
reads process memory. Build from source if you would rather not
take a binary on trust.
See CHANGELOG.md for what is new and THIRD_PARTY_NOTICES.md for
licensing (Signature Lab is MIT).