Tip #1 Don’t start with “Mujhe shaadi nahi karni”
Assuming that your parents are important to you and that you do want them to be happy, it might be best to sit down with them and have a serious talk. Explain that you agree with them that marriage is important, but you want to wait right now and take your time.
If you and your parents come from a culture where they are expected to play a part in the search for your life partner, then share what you're looking for and what matters to you. Emphasize that you want to get to know the person first because you want to be sure that you truly are compatible before you complicate matters with sex (which marriage inevitably does) or dating.
Then allow them to express their concerns and point of view. Listen to these concerns with undivided attention and don’t rebut them until they’ve expressed all of them. Then, thank them for their suggestions and address only those issues that they’ve gotten completely wrong. Tackle only those that create misinterpretation of who you are and whom do you want to be with.
Golden Tip - Do yourself a favour and stay away from words like, “You’re wrong.” Are we clear on that? Good and don’t blame them. They only want the best for you, right? (If they don’t, that’s a whole different issue altogether).
If they’re critical of you being picky, point out that marriage is a long time to be miserable together and remind them that you’re the kind of person who takes time and tries something out before taking a decision. You are, aren’t you? Or you’d be married already.
They may have made their marriage work on the fly, but you do things differently and that’s okay too. This might need some time for it to sink in, so don't be too disappointed if they don't react the way you hope, immediately. But they’ll eventually understand and accept this. Isn’t that all you want - some time and kindness?