Hi Steve
By Car or Train?
By car pretty straighforwrd (I think)
take M11 southbound all the way to A406 north circular,stay left onto the A406 towards Redbridge Roundabout stay with it you will come to a junction with the A13/A1020.Take the 2nd exit sign posted CITY AIRPORT/ WOOLWICH FERRY keep going you'll come to another roundabout,take the 3rd exit CITY AIRPORT keep going straight this is the A1020 follow this until you get to another roundabout that is the City Airport roundabout,keep heading towards the Airport at the entrance to the Airport on the roundabout take the 2nd exit (you'll go past Silvertown Station) At the junction with A117 go straight across onto Gallians roundabout and take 1st exit onto Royal Albert Way stay on this until you reach Royal Victoria Dock parking
By Train.
Stanstead express to Liverpool Street,circle line to Tower Gateway,then dock lands light railway to Custom House
If I were you I would go onto Stansted Web Site they will have other info there

Good Luck