Right, got it now. When you get off the train just follow the directions in reverse. You'll have to get a ticket and go from Percy Main Metro to North Shields. At North Shields there is a free bus down to the ferry terminal (ferryLink 333) which stops at one of the bus stops opposite The Neville pub on Rudyard Street (see
here).
Once you get to South Shields on the ferry, walk towards the Metro station and you should be able to get the 35 from Keppel Street bus station. Either that or if you buy a 2 zone Daysaver at Percy Main, you'll get on the ferry for free and then you have the choice of getting the Metro to Pelaw to change for Sunderland or even going to Heworth and getting the Northern Rail service from there to Sunderland! Of course all depends on the time of the trains, although the journey time looks to be a bit longer from South Shields to Sunderland by Metro/Rail than bus. So if you have a bit of extra time you might consider just using the Metro to get to Sunderland.