I have been studying along these lines too and here are somethings that I have picked up. Criticism from masters in this are welcome.
First, watch this... he does courses in this if you are up to investing the money.
I would also recommend reading the whole Kindle series by "The Rogue Hypnotist" (a different person). Each book costs about $1 but are much more readable and informative than many more expensive books.
I think the key thing is that your girlfriend has to believe that you are a hypnotist for her to accept the suggestions. I would try suggesting relaxation and keep practicing over various sessions until you can build a deep trance. As xfeeefeee mentions, start off as fantasy pretend play because everyone can believe in that and then the words will be more believable. Once physical feelings are generated as a result of imagination then the pretend play has become real.
The other key thing is suggest physical feelings. e.g. If you suggest to your girlfriend that she can feel some warmth or energy in her chest which is moving downwards, she will feel something. If someone concentrates on any area of their body then they'll start to imagine that there really is something going on there. If that imaginary feeling is moving south then you are getting close to generating feelings in areas associated with sexual arousal. Try this for yourself. Rehearse how you would suggest this to someone. If you start to get turned on, even slightly, then someone else can experience that too.
After you have read about the basics, try and deconstruct why they are saying what they are saying. Look at the erotic hypnotists, erotic hands free hypnosis and understand what they are trying to do. Look and deconstruct Speed Seduction videos. After a while you can see the overall patterns which are not apparent and even see when people are seeking to induce hypnosis just through conversation.
This book Covert Hypnosis: Easily Sneak Your Ideas Into Their Mind by George Hutton is probably rehashing other NLP books but is cheap and a good guide to hypnotic language patterns.