How does customisation affect damage?

Obviously reading the description, customisation adds 20% damage to a weapons damage, but when and to what damage does this get added?

Does it get applied before or after other mods?

Does it only affect damage dealt by a normal attack with that weapon, or does it affect the damage dealt by skills, spells etc?

I think it's combined with other % buffs like 15^50 so it's 35%. And i'm sure it adds that damage to normal attacks.

It only applies to the damage done by the weapon, not the damage done by skills. If a skill says, for example, that it does "+20" damage, that +20 is applied after the base damage (including the +20%) is calculated. In other words, it stays as +20 and doesn't become +24 damage.

Edit, btw, I don't know the exact method by which mod bonuses are calculated (but the info is available somewhere I think), but the +20% customize bonus does stack with other bonuses such as "+15% while health is above 50%" (15^50).

It should stack with 15^50 (haven't tested it so can't confirm it for sure). I dont think it adds up for 15% +20% it would probably be something like (1.15*damage)*1.20 or the opposite.

The damage dealt by attack skills is always armour ignoring. If you use an attack that deals +24 damage with a Candy Cane weapon, it will deal 34 damage (10 + 24). If your weapon would normally deal 0 damage, you will still hit for 24 damage (unless they're using something like [wiki]Shield of Absorption[/wiki]).

As such, things like customization and inscriptions do nothing to the bonus damage of attack skills.