For one thing, the average skilled medieval blacksmith wouldn't be making armour, especially not plate - that's a job for a proper armoursmith, who may be working in a city guild or royal armoury. Second...you may need your mage or setting-equivalent to pitch in.

For all things armour-related, we of course turn to u/WARitter's many posts on this matter, in particular:

• On the steps in manufacturing armour, so's your smith knows what they're getting into;
• On the process of making 'munition' armour, the cheap stuff;
• and on the process of making mail armour, as a comparison. Depending on the time period you're pillaging from and the style of the plate your smith is making, mail inserts covering vulnerable areas may be necessary.

I also recommend their flair profile for more armour-related posts. And as always, if anyone else would like to type up an answer of their own, please don't let this linkdrop stop you!