Declaring an attribute for a different namespace in XML Schema

Declaring an attribute for a different namespace in XML Schema

I've been using an XML format that is a mix of different existing formats
and some custom elements and attributes, and I thought I should write a
schema for those custom bits.
One thing I do is use custom attributes on elements in existing formats,
like this:
<ns1:something attA="b" attB="a" ns2:extraAtt="c"/>
I understand that doing this is allowed but I cannot think how to declare
my "extraAtt" in XML Schema or, worse, in a DTD.
I have tried reading the specification, but it could just as well be
written in Chinese as far as I am concerned. Most tutorials talk only
about "name", "type", and "use", e.g. this one and that one.