I've always had the idea of creating a skin knocking around in my head, but sadly time is always a limiting factor. |
If you don't have time for a WIndowBlinds (assuming that is the skin you're referring to) you probably don't have time for an IconPack. 
But Morphium is correct,.... start by making the larger size in PSP, PS, Corel, or what ever, save as png, and import to IconWorkshop (my choice) or MicroAngelo, or maybe one of the free icon editors. I'm not sure how good the free ones are at this (never tried any of them).
I'm not sure the latest SkinStudio makes the process any easier, and I have to wonder why Morphium would say that, and then admit he's never made any icons.
As a general rule, images will downsize much better than upsize. But also keep in mind that png with alpha transparency, usually don't look so good when converted to icons for any OS other than XP. So you may have to make 2 versions of the png if you want to create icons for all the OS's. One with alpha, and one without. 