Walk to North Greenwich Station. (No more than 10 minutes stroll).
Take bus number 188. (Buses run every 8 minutes. North Greenwich station is the terminus, so you don't need to worry about which direction the bus is travelling).
Alight at the Trafalgar Estate stop. (Simply tell the driver that you want the stop for the National Maritime Museum. Scheduled journey time 19 minutes maximum, often a bit quicker).
Walk to the NMM. (Just a few minutes stroll).
Chris