I should have some "instructions" at w:User:Fewfre/signature.css. However, you may be better off reading w:c:tardis:Help:Signature (something I sadly came across after figuring out most of the problems myself).
A question though: I created my customized signature in this Wiki only. When I created it in another wiki it shows 'Template loop detected: User:MouseyLady/sig
I'd have to see it to know exactly why that happens, but odds are on User:MouseyLady/sig your calling {{User:MouseyLady/sig}}.
And yes, sadly you will need to create a copy of your signature for every wiki you visit. This can be handy if you want to personalize it for that wiki (ex: a lot of RuneScape sigs have RuneScape items on it, plus the RuneScape wiki allows you to do hover effects, which can only be taken advantage of on a custom sig).
