As above, may as well change the whole inner and outer cable while you’re doing it unless it’s very new. It’s one of those jobs that you realize once you’ve done it that you’ve put off far too long.
For your original question, depends on the bike.
MTB’s tend to have adjusters on the shifters, so no point adding it in the loop from the handlebars, may as well put it next to the rear mech.
Roads bikes don’t have adjusters on the shifters so makes more sense to put the adjuster either directly on the lever (older shifters where the cable exists outside the tape) or under the stem (newer ones with hidden cables). I’ve got some nice neat Ritchey ones that plug directly into the shifter which is great as I can trim the mech without even taking my hands off the levers.
What combination of shifters and mech are you running that has no adjusters?
will I need to fit a new cable inner as surly by adding the adjuster it’s making the outta longer ? Or do I cut out roughly the same as the adjuster if that makes senses ?
A little bit of both. Unless the cable is very straight you’ll probably find that the curve needs to be a few mm more or less to get a nice bend. Road bikes are generally way more picky about this though with short cable runs and tight bends. MTB’s have enough length that a few cm of extra cable doesn’t matter.