The snap function will work with pre-existing features that you have in a plan or in conjunction with the interactive base map function. More detail on this can be located here.
By default the snap function is always on and when it's blue you know that it's active.
Go to Draw select your drawing tool of choice.
Move your cursor to your starting point and you will see that it is magnetised to the edge of the feature. single click to start drawing your feature. Move your cursor around the feature and it will snap to the edge. Double click to finish and click on Finish.
