Combining nodes

Hi all.

So I need to take a bunch of nodes in one part of my input xml, and some
nodes in a different part of the xml, and still some more from another
part of the input, filter them on differing criteria, pick out different
fields and output a simple of nodes that looks nothing like the input
nodes.

Each of the set of input nodes are in different formats, and the filter
criteria vary.

How the heck do I do this?