Applied online. The process took 2+ months. First, received a coding challenge to do in Java (complete 1 out of 2 problems), and a questionnaire. You are given 10 days to complete them with an option to extend the deadline. I submitted mine after 9 days. Questions are either interesting or silly, depending on how you look at it. I'd advise you to spend some time to think about the answers. The coding challenge was interesting. I had little Java experience, so most of my time was spent figuring out how to do things in Java. There is a post online, written by a ThoughtWorker, about how he assesses coding challenges. It was really helpful. Just google it. And, do include unit tests. If you read any article from ThoughtWorks, they always mention TDD, so keep that in mind.
After about 2 weeks, I received an email, inviting me to schedule a skype call with a recruiter. The closest day to schedule was in 2 weeks, so that was the longest break between the steps. The recruiter was very nice and the questions were almost 100% behavioral. I was asked a few questions about the coding challenge, along the lines of what would I do differently, how would I improve it, but the rest of the questions were about me, why I want to join TW, what does social justice means to me etc. At the end of the call, I was given some feedback on my coding challenge, which was mostly positive.
In a few days, I was invited to the next step -- onsite interviews at the NYC office. Despite being nervous, it was BY FAR the best interviewing experience I've had. All the people I talked to were super smart and really nice. While it was a long day, all the interviews flew by really quickly. I also took a few timed tests. If you are not familiar with Wonderlic test (like me), do yourself a favor and google it. There are plenty of practice tests. It is not exactly hard, but being timed and nervous can play some tricks on you. The other test was TW own logic test. If you google it, you can find some past examples of the test to know what to expect. In general, I had a really positive experience with the whole interviewing process at TW.