The secret for the standard rehabber is estimating isn't really estimating. Estimating is the professional title but that's not really what you're doing. You're learning how to scope damage. Scoping damage is looking at a room and being able to capture the data on what is damaged, what are its measurements and does it need to be repaired or can it be fixed in place. Common sense will answer 95% of this for you but that 5% can bite you. Defer to experts on anything involving electrical, hvac and plumbing that isn't blatantly obvious. Trust your gut too on anything that doesn't look right, i.e. cracks in stucco.
Once you know how to scope, estimating is no more complicated than multiplication with a unit price reference guide like Hometech, Walker, or Xactimate to guide you. If you need 200 square feet of drywall replaced, they will tell you what the price is to put down tarps, replace those 200 square feet and repaint the affected rooms. Personally I recommend Hometech. It's a lot easier to use for a beginner than Walker and a heck of a lot cheaper than Xactimate. Now I personally use Xactimate, Walker, Symbility and sometimes even Bluebook (yes there is a blue book for estimating rehabs) but estimating is my business. And yes you can hire a professional estimator but just be aware they range between $125 and $250 an hour.
Caveat, as with any profession that involves a professional opinion you're going to run across people who are more and less expensive than the book. You'll have to adjust to that. That just comes with time.