Idioms can be a bit challenging to learn, but here are some tips to do it more effectively:
Go beyond the meaning. Try to understand the context where the idiom appears. Think of other situations where it can be applied and come up with your own example. This way, you'll remember it better.
Create an image in your mind. For example, for "I'm all ears," imagine a man with rabbit ears. For sure, this man will never miss a word you say because all of his attention is dedicated to you.
Explore its origin. For instance, "Happy as Larry" can be traced back to a newspaper headline in New Zealand which reported about Larry Foley winning $150,000 in a boxing match in the 1890s. With this information, the idiom will make more sense to you.
With these tips, not only will the meaning of the idioms be clearer, but you will also remember them much better.