as easy as falling off a log
To me this work is as easy as falling off a log.
He is a maths professor, so for him to help you with your homework will be as easy as falling off a log.
It is a piece of cake for him to properly install any computer part.
The expression as easy as falling off a log alludes to standing on a log that is moving downstream, a feat in which falling off is a lot easier than remaining upright.
The expression 易如反掌 literally means as easy as turning one's palm over. It only applies to situations where the person is a skilled practitioner in the relevant field of undertaking.