Generally, a tailor can take the waist in of any pants about 1" linearly / 2" circumference smaller, without needing to do any "interesting" work and without fucking up the fit. Up to 2//4" is often possible without recutting, but as you go above a 1//2" reduction recutting may become necessary. On the flip side, pants can generally be let out up to 1//2" inches without changing the shape if there is enough material allowance to do so; more is rarely possible.
What that means is that if you buy trousers and they're a little too big, you bring them to a tailor and pay about $20 and they make them smaller for you. A little too little, same thing, usually.
My perfect fit is 16.00-16.25" and I can get 17" down easily. My body shape is not interesting enough to require custom-fit, and trousers are way simpler to fit than jackets so most people don't need custom-fit.
Now that said, there are some overlaps in price between high quality RTW trousers, and good quality MTM, where you may feel you'd rather dial in an MTM pattern and get the "perfect fit" rather than pay someone for their brand strength and still need to do mild alterations. Many people do, so I won't say you shouldn't get MTM pants. I will say that online MTM will take several iterations to dial in ($$$) and unless you need it for shape reasons or want it for ... want reasons ... you are generally best off buying RTW and tailoring it.
To answer your very specific question, online MTM will tell you how to measure yourself and how to turn that into an order. Remember that there is no one perfect fit or perfect number, even for an individual, so going from a 32.5" waist to an MTM order is not as trivial as 'click it and ship it' unless you're getting the most default option and never deviating from it.