Are you adding length or just adding a hinge?
Select hinge kit based on tongue size, weight rating -
https://www.etrailer.com/s.aspx?qry=tongue+hingeTheir directions are really good.
From memory -
Adding new tongue section to existing cut off coupler so you have a clean end of tongue. **
***For adding a hinge into an existing section of tongue - Mark line on all four sides and cut with reciprocating saw. Debure edges.
Remove safety chains.
Dry fit hinge - mark hole locations.
Dimple hole centers.
Drill all 4 holes.
Debur holes
Install hinge, bolts and tighten nuts
New tongue section:
Dry fit hinge - mark hole locations.
Dimple hole centers.
Drill all 4 holes.
Debur holes
Install hinge, bolts and tighten nuts
Dry fit coupler
Dimple hole centers.
Drill all 4 holes.
Debur holes, install bolts and nuts
Select safety chain/cable location
Dimple hole centers
Drill holes
Install chains/cable, washers nuts and bolts.
Install trailer harness extension.
https://www.etrailer.com/Trailer-Wiring/Hopkins/HM48145.htmlInstall video -
https://www.etrailer.com/Folding-Trailer-Tongue/Fulton/FHDPB230101.html