Extending a gable roof can be tricky since it involves working on the roof but you may be able to do it yourself if you only need to add 1 2 2 inches 1 3 5 1 cm.
How to extend gable end roof overhang.
There will be more work involved weaving the new shingles into the old but the process is really the same.
Most discussions i have seen on extending a rake suggest cutting back the existing roof sheathing for one or two rafter bays and cutting new sheathing to span the extension and the cutback.
How to extend gable end roof overhang for the arrangement of the gable end overhang for the standard roof you lengthen the ridge beam beyond the facade for previously verified length using cut rafters.
Your wrb should connect or overlap with your roof underlayment.