It's not easy to balance your hobby projects with a full-time job, you'll have to find what works for you. Some people chisel away at it in pieces with whatever free time they've got, others will make that time themselves. I usually keep a notebook around to jot things into during the workday, so the next time I have an opportunity to work on it, I have the details ready for what I'd like to focus on.
Depending on what you're trying to make, you could try working on simpler designs that take less time and effort, for practice. You'll learn what works and what problems can crop up, giving you a better handle on what's needed and how to manage your time.