| Blymurkla |
Are you enchanting your armor yourself? The crafting rules are rather complex and I'm not the right person to help you with them.
If you aren't enchanting yourself, you'll have to find someone to do it for you. This will rely on your GM. In certain campaigns, it might just be as simple as going into the nearest Magic Shop or asking a Wizard's Guild an they'll direct you to a competent crafter who does the job for you. You pay the requested sum, get your armor back, no worries.
If your party is nowhere near a town where crafters are available or if your campaign is low-magic or imposes other restrictions on magic items, I imagine this can turn out into a sub-quest.
Anyhow, if you pay someone to enchant your armour for you the going price for a simple +1 to AC is 1000 gp. You can only enchant items of masterwork quality, but you're in the clear, because mithral is always considered masterwork.