이렇게: this way, like this (demonstrating how it is supposed to be).
이대로: as it is now, the way it is now, maintaining something that is already established.
- 이번엔 이렇게 해 봐: This time try it this way.
- 이대로 끝내야 해: I have to finish it as it is (without trying anything new or different).
* There's also 그렇게 and 그대로 which are similar (이: this, 그: that).