Stop immediately and get an expert in to evaluate your grading. In this case, you haven't given us much context on what's around that, so you would risk diverting water in a potentially unpleasant direction if you did that yourself.

Now...if you have a large property, that's in the middle of it, and all you need to do is divert the water under the walkway, you might be able to install a French drain under it yourself. I'd still be inclined to get an expert's opinion, at the very least, to assure I weren't causing more problems than I'm solving.