Can you please suggest an effective way to memorise the idioms?
(not a professional teacher) I would suggest that there is no easy way to memorise idioms. The best, and possibly only, way to learn them is through use. If you encounter an idiom you are unfamiliar with, search out its meaning, then try using it in the appropriate context. Make sure that you do find out what the appropriate context is, though, as idoms in any language can cause offence if used inappropriately.