When you put on the Winn grip handles can you just push/force them on like eva foam, or do you have to ream them like cork?

Half and half. Ream more than foam but a tighter fit than cork. They'll stretch s little but you'll tear the skin off if you pull too hard. 

I like Winn grips, in use. They're light, very comfortable, and provide a secure grip when wet. And, they are interesting in appearance. All positive attributes.

But, they are difficult to fit and install. I've put them on four rods so far. I've bought grips for seven. Three ended up in the trash because I damaged them; either during reaming, or during the install. Didn't tear the skin, but I wrinkled the bejesus out of "em.

I'm truly thinking I'll never use them again.

Hey guys I have another question about the grips. How do you ream the butt grip? Does it go all the way through and then you out on an end cap like cork?

Many of their grips have tenons on which their end caps may be glued.  On the butt knobs, don't ream all the way through, just ream to about 3/4 of the length of the knob.  For your specific question, I think you want a full grip finished off nicely, right?  In this case ream carefully, not generating a lot of torque, with your Dream Reamer or equivalent to get the ID you want, a fairly easy slide fit over the blank.  Remember what Ghoti says about skin wrinkling.  The grip you should be using will have the previously mentioned tenon.  Glue on the appropriate Winn butt knob to finish it.  Mudhole offers much of the Winn line-up.