When you have decided that you want to take a more active part in improving SoapUI then there are a few things you have to do before you can get started.
Contributors Agreement
First you have to send us a signed copy of our contributor agreement. This is mandatory for both our sakes and a standard thing that all similar projects use. The purpose of the agreement is to assert both your rights as an author and our rights as an intellectual property holder. When we have received the agreement and all is ok we can start accepting your patches.
Contributor Agreement Checksum
Branching scheme
Now you can fork the project on GitHub and create a pull request with your changes from the next branch. This is our development branch for the next release so, create your branch from next. Keep the patches focused and self contained on a topic branch with tests and clear commit messages. Be ready to defend your changes and don't be offended if a fix is rejected at first. Just discuss it and perhaps you can convince us that your code merits inclusion. Having a good motivation and explanation attached to the pull request will make our job in the receiving end of the pull request easier and increases the chance of your patch being accepted.
Testing
Make sure that you have sufficient test coverage and bear in mind possible regressions. Try to see if a changed code section is used somewhere else and if so add a regression test proving that your change didn't introduce any regressions. This is of course impossible and in a perfect world there would already be 100% test coverage and you would know right away with a high degree of certainty that all is ok. In general, more tests is always better and will make it more likely for your patch to be accepted.
Have fun coding!