There are a number of ways to install a porch railing system. Depending on the style and materials and 'existing' structure(s), the installation will vary. So, I don't have much to give you in that respect.
However, one idea is for YOU to come up with plans on how you envision the installation taking place. Do you want nails or screws? Do you want them 'hidden'? Do you want 'X' to be centered? Etc., etc.. Another idea is to find a porch somewhere that 'looks' like yours and beg the homeowners to let you look at it (explaining to them, of course, what you're going through.) At the same time, you've held back the final payment to your carpenter AND you're told him to stay away until you get your 'plans' together. You see, he's gonna say, "Either let me finish how I know to finish or give me your plans." I wouldn't waste any time. I think you should have plans in no more than 2 weeks. This means you need to dedicate more time than just weekends to do your work. Also, there are books in libraries and book stores. Spend a night out looking into BOTH places. I think it's premature to keep whatever $$$ you owe the carpenter and get someone else because he needs an opportunity to 'fix' his mistakes. (At least I'd say that would be a good idea if I was the judge presiding over his lawsuit in which he sued you for non-payment.) If he has all his $$$, you'd better get him plans ASAP. If he doesn't fix the problem, you'll need to sue. SO, at this point, I say take some pictures of the current workmanship! Start a LOG of your 'thoughts' and efforts to make this right. Document your phone calls as well and what HE says he's going or not going to do. Again, if you end up in court, if I was the judge, I'd want to see what efforts YOU put into this to make this job right!
So get crackin'!!! My best to ya and hope this helps.