I'll give you some hints – see if you can work out the meaning. If you post your suggestions, I'll come back and check them for you.
1. Do not sell him short.
— You're "selling" him something, but not in a literal way. And you're selling him "short". Do you think "short" is negative or positive?
2. Left holding the bag.
— Who is usually left holding the bag? Is it something people want to do?
3. For a song.
— How much is a song worth?
4. Bend over backward.
— The full phrase is usually something like "I bend over backwards for you – and this is all the thanks I get!". Do you usually bend over backward? Is it comfortable?
5. Take the bull by the horns.
— What kind of person takes a bull by the horns?