Download the release archive and extract it into your installation. The src/addons/MC/ContextualSigs/ directory should sit alongside the other add-ons.
Navigate to AdminCP > Add-ons and click Install next to [MC] Contextual Signatures. The add-on creates its own signature and forum-mapping tables.
Visit AdminCP > Setup > Options > [MC] Contextual Signatures. Set Max contextual signatures per user to a sensible cap and decide whether mappings should inherit through the node tree.
Optionally pick forums to exclude under Excluded forums. Posts in those forums always show the user's default signature.
Navigate to AdminCP > Groups & permissions > Permissions and grant Use contextual signatures to the groups that should be allowed to use the feature. Optionally grant Bypass maximum contextual signature limit to staff or premium groups.
Confirm the add-on is Active at AdminCP > Add-ons.
Visit Account > Signature on a member account that holds the Use contextual signatures permission. The page should now offer Add another signature alongside the standard signature editor.