one of my players is going on about wanting to craft armour but I cant find anything in the rulebook to suggest how to go about it, any help?

Take a look at DH1 "The Inquisitor's Handbook", crafting rules are on p.245-248.

In the Only War Core Rulebook p.115 is devoted to the Crafting "Sidebar", and lets you use Medicae, Survival and Tech-Use to craft things.

For armour you'll need Survival or Tech-Use, although with Survival I would not allow a player to craft anything but leather armour or similar. I might let them fix a set of guard flak by patching it with scavenged bits though.

So, Tech-Use based armour.
That takes, as a guideline, 1d10 days and 12 successes on an extended test. Materials are acquired through a logistics test which is one step easier than acquiring the armour would be (so Common -> Plentiful, Very Rare -> Rare, etc.).

However:

[...] many items created using this Skill are fiendishly complex or their design a closely guarded secret and are only available to characters with the right knowledge and access to advanced workshops.

Leading me to doubt whether your guardsman could create Carapace.