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.
1. Start by well lubricating the outside of her pussy, rubbing her clit, and slipping one finger gently inside her. Add another finger, keep the lube handy and add more as needed. Be patient, allow her body to adjust as you add a 3rd finger, adding more lube, keeping your pace steady and even. Gently tapping her clit with your thumb as you finger fuck her will add to her relaxation and pleasure. You sitting between her legs and her on her back is the best position to start this process.
2. Let her body be your guide. She's likely to begin rocking her hips into your hand, giving you a sign that she's ready to progress to a 4th finger. As you slide the fourth finger into her, hold your hand in place for a few moments, then slip your fingers further into her, keeping your knuckles facing down. Her pelvic bone won't appreciate your knuckles smacking against it as her muscles haven't become loose enough for them to stretch enough to keep your knuckles off her pelvic bone.
3. Once you have your fingers fully inserted into her, gently nudge your knuckles in her, you may find twisting a little with your hand will help this, turning it from side to side, keeping the knuckles facing down or to the left or right, not towards the pelvic bone yet! Once you've managed to get your entire hand inside her, the only thing left to do, is add your thumb. Again, let her body be your guide. Ladies, do NOT be afraid to reach down and hold his hand back or speak up and tell him it's too much or you aren't ready. Fisting doesn't always happen the first time, especially if your partner has larger hands.