Everyone says "follow your passion". Fewer say "keep your job while you test it". Here's a timeline that actually works.
Month 1-3: Validate. Talk to at least 10 people who could be customers. Don't ask "do you think my idea is good?" (everyone says yes to be nice). Ask "would you pay X for this?" That's a completely different question.
Month 3-6: First customers. Register your business. Do your first jobs evenings and weekends. Send invoices. Learn bookkeeping. It's not glamorous but it's necessary.
Month 6-12: Build routine. You should now have 3-5 recurring customers or a steady flow of new ones. Get a website (five pages is enough). Start showing up on Google.
Month 12-18: Evaluate. Are you earning enough for it to be worth it? Can you see yourself doing this full time? If yes: plan the transition. If no: you've learned a lot and it hasn't cost you your job.
The beauty of starting on the side is that you never risk everything. Worst case, you learn something new.