There are many variables. First is your location. What do you have access to? Second is what do you expect to achieve from the process. Cloth selection, style variations, better fit, etc.?
You can find MTM online. You supply the measurements. Risky. You will be unsure of the silhouette and what it will look like on you. Risky. Measurements provide direction to the maker but there is interpretation and posture issues that need to be accounted for. Risky. You may not actually see and feel the cloth beforehand. Risky. It is all done in cyberspace with no or little human contact.
If available, you can work with a direct seller or an actual tailor who takes your measurements and the clothes are made at a CTM house. Use search function if you need CTM house explained. You work with someone who sells MTM suits from a store front, tailor shop or comes to your office or home. Some are very good at this. The best ones understand how it works and use fitting jackets that are the same cut as the pattern that will be used to cut your suit so you can see how the lines of the cut work with your body type. Not all do this and someone who really understands the pattern used and knows how to measure and fit really well will be less dependent on using sample garments. If they are using a top notch CTM house there will be multiple silhouettes for you to choose from or for the sales associate to prescribe for your needs. It is easier to assess what adjustments are needed this way because they are seeing a finished product on you and don't rely entirely on their own eye and experience even though assessing the adjustments is better when the guy has years of experience. This type of person will also have various lines of sample books for you to choose from. You will understand why this is good if you read the next paragraph.
In this scenario, If you know more than he does, it won't end well. Moderate risk here. If he works without sample jackets or has no real experience as a fitter the risk increases.
You can use a RTW line from a men's store or department store that will do MTM. You will get to choose the brand you like and the suit that comes closest to fitting you as a base. Price range will vary by brand. You will for sure know what the silhouette is and be able to try on a garment in the same cut as they will make for you. You may be limited to the factories methods of detailing as this will be made in the most commercial process and limited to whatever templates are used. If you want 2 1/2" pocket flaps but they are only programed to make 2" pocket flaps, you are out of luck. Get the idea? You may be limited to what the maker has in inventory for cloth unless they accept outside cloth. You have to ask. Also, garbage in/garbage out. Results depend on the skill of the salesperson or tailor that measures you and communicates with the maker. If the sales person speaks English and the maker Italian, this is another issue. All this applies to the direct seller too. Moderate risk.
On all three scenarios there will be vendor specific variations, sales person variations based on their point of view and knowledge and there is also a little unknown at work due to the logistics of the process. Most outfits will have reviews and user experiences you can find online to help you determine who is worth working with. Take reviews with a grain of salt.
If you take what I have written the right way, it will get you to ask some of the right questions. Asking good questions is how/where you start the process.
As despos said above you are best off with a live person who can measure you as opposed to online (which very rarely works out well). There are a few people here on SF that do it depending on your location. where are you located?
If you are willing to go to Buffalo and like traditional (Ivy) clothing,
my guess is that O'Connells might be able to do MTM through one of their
vendors:Samuelsohn, Hilton, Southwick. They offer a very wide selection.
I've never bought from them- I am in California. But I've spoken to them by
phone and discussed MTM. I like the natural shoulder Ivy style.
My only "customization" is side vents and a nipped waist on an otherwise
trad garment. I am also lucky in being able to wear jackets from numerous
vendors with minimal or even no alterations.