I told mine very early in the relationship when she asked if I have any fetishes. I showed pictures of women and couples, no solo men. (Nylonmichelle's pictures/clips are a good starting point). I have since backed off of the encasement aspect but have routinely politely asked her to her pantyhose. She's happily obliges it easy for her because she wears them anyway already. Like yours, she likes that she can do something so simple that excites me in public. It builds. I'm not pushing the encasement because I want her to feel completely comfortable with me i.e.- her telling me about her kinks and watching her preferred adult materials (BDSM) with her and alternating with non-encasement nylon fetish DVD's. It's a process.

I have had a former girlfriend who once volunteered to try encasement about a week and a half after telling her about it. It depends on the girl/woman. Play off her reactions to it. Some girls will jump right into it while others will take time to get to that level of comfort. Remember that one of the primary driving forces in female psychology is their comfort level. Part of that is familiarity.

Sort of work it into conversation. Be prepared - bookmark/save pics and/or clips that you want to show her as examples of what you're talking about. Remember that pantyhose is just a common garment for women, similar to socks or boxers/briefs for men. Most won't immediately get that it's a fetish object for you. Your girlfriend already gets this basic aspect about you and has accepted this about you and that is wonderful. She sounds like she has lot of positive qualities.

Don't rush. It's like having sex with a woman. If you're in a rush and try to force it, it's much less fun than when you take your time and relax and let her arousal build. She'll wrap her mind around it in time. You have no control over that time frame, though. However, just like when you make love to her, if you take it easy and let things build after telling her, good things will happen.