Let's start off by saying, don't believe everything you seen in a porn video. The man manages to slip his hand right in on the first try? No, not quite. Remember that's a porn video, those ladies are primed and ready to go and have practice. Their muscles are used to the torture and torment of the initial penetration, it's unlikely your partner is accustom to that - yet!
I can not stress enough that patience is a virtue. You must be a patient partner, the female body is a marvelous thing. It can be stretched and contorted to give birth, so if you are patient and she is as well, you can "train" her body to accept your entire hand inside her, and if done right, her orgasms from being fisted will be like a rapture throughout her entire body.
The first step is finding the correct lubrication. I recommend a good water based lube, there's many available in your local adult shop, or on line through many sites. I also recommend using non latex surgical gloves, however, some people are not comfortable using gloves and want the skin to skin contact. In that case, please remember to wash your hands thoroughly (both of them) with an anti-bacterial soap, avoid simply using hand sanitizers, they contain a large amount of alcohol which can sting and cause the wrong sort of pain. A towel placed under her bum is also recommended, this can get very messy, not only from the lube, but the multiple orgasms you're going to experience with her.
Begin the same way you normally would for any sexual encounter with your partner. Kissing, fondling, etc., bonding with the person. If you've never fisted someone before, or your partner hasn't been fisted before, it's essential that you bond throughout the process. This is a very personal experience, she's allowing you to do something to her that's very intense and can be very painful if forced. Take your time in foreplay, but if you've read any of my other articles you already know foreplay is always stressed in love making.