Can I sign up now but only practice and take my interview later?

Absolutely! You can sign up now and come back later, at your convenience, to practice and take your interview. You can even start your interview and pause between each question. When you return to the interview interface via the link, the interview restarts from the question following the one you recorded last. Finally, keep in mind that once a question has been started, you need to record it all at once. No second chance!