There are a number of ways in which you can secure the G spot stimulation so that she gets an incredible g spot orgasm. Here are a number of positions you can try:
1) A rear entry always works well for a G spot orgasm. You can try this from a standing or lying down position. Just make sure that her head is not lying on the bed if you are standing up. Try and get her hips to arch a bit when you are entering her. Your penis should try and hit the vaginal wall. This way, you will be able to access the g spot easily!
2) The woman on top position is great for G spot stimulation. This is because she is in control and therefore better able to angle her hips so that she gets the most pleasure. You can try the cowgirl position and its variations for getting more access to her g spot. She can lean back at times in order to get more penetration!
3) The rotating woman position is also great for G spot sex. Just lie down flat and have her straddling you. She should be touching your ankles in this position. She can then rotate her hips in a rough 8 movement so that her vagina is tightened up and the penis is rubbing against it.
4) Another tantric variation which makes for great G spot sex includes the flower press position. In this position, her feet should be on your shoulders or your chest. Start off by having a missionary position and then have her lean back and put her legs over your shoulders or against your chest. This is excellent for g spot penetration because the penis is pressing against her at the right angle.