There have been a lot of "how do I become a contractor/consultant" threads on HN in the past. Most of the advice isn't specific to infra, so searching for those will be a good starting point.

To summarize: the best way to get started is always within your network. Reach out to people you know who might need your services, and ask them if they do. If those work out, they'll refer you to other people they know. If things go well, you'll be able to keep yourself busy through word of mouth and never have to advertise.

There's a lot of debate over whether to charge by the hour, the day, the week, or by project. If you're just dipping your toes in the water, it doesn't hurt to start with hourly, because it's low risk for both parties involved. Here's a heuristic: start with the hourly rate you're making at your current full-time gig, and double it. That accounts for the fact there's overhead in contracting that isn't as visible to you as an employee in a full-time job (insurance, employment taxes, etc.), and also for the fact that you may not be able to keep yourself employed full time initially, so it gives you some wiggle room.

If your first client doesn't balk at the rate, increase it by 10-25% for the next client, and repeat. Stop when people freak out at your price or you're retired, whichever comes first.