Any tailor with any experience on the job is going to have zero issue and best they know the exact purpose of your request. They may offer some insights/options you my not have considered.
Up front, make sure you are dealing with an actual tailor though and not just someone who does alterations. They will be the most informative and set you up with the "extras" you are likely going to want. Like reinforcing the area you plan to carry in, possibly waistband grips to help your shirt stay put (and remembering your shirt will also need to be sized for carry), etc.
I think it's pretty common to just wear your EDC and/or mag holder as you normally would for measuring. Semi-autos, take in an empty mag to replace your loaded mag after clearing it for measuring.
One other thing to consider is if you are often in suituation where you are unable to carry... you'll want another set of pants fitted for non carry or alternative carry days. IE., OWB, shoulder...
Up front, make sure you are dealing with an actual tailor though and not just someone who does alterations. They will be the most informative and set you up with the "extras" you are likely going to want. Like reinforcing the area you plan to carry in, possibly waistband grips to help your shirt stay put (and remembering your shirt will also need to be sized for carry), etc.
I think it's pretty common to just wear your EDC and/or mag holder as you normally would for measuring. Semi-autos, take in an empty mag to replace your loaded mag after clearing it for measuring.
One other thing to consider is if you are often in suituation where you are unable to carry... you'll want another set of pants fitted for non carry or alternative carry days. IE., OWB, shoulder...