Dude31 the first consideration would be positioning , depending on her comfort level you can use the lithotomy position, lateral decubitus, or Sim's position or the knee–chest proctologic position.
The lithotomy position, the patient is supine with the legs drawn in toward the trunk and the knees allowed to fall out to the side. The lateral decubitus, or Sim's position, the patient lies on the left side with the buttocks near the edge of the examining table or bedside with the right knee and hip in slight flexion. The knee-chest position the patient rests on the knees and chest with head is turned to one side, arms extended on the bed, and elbows flexed and resting so that they partially bear the patient's weight; the abdomen remains unsupported, though a small pillow may be placed under the chest.
Images can be seen here http://medical-dictionary.thefreedictionary.com/position
Dude31 the most important factor is to be gentle, take your time and not rush in your excitement, stop immediately if she experiencing any pain or discomfort. Its always a good idea to have her empty her bowels beforehand.
As for the procedure itself:
1. Place a non latex glove on ( remember latex allergies can be life threatening).
2. Put a lot of lubricant on the index finger of your right hand.
3. Tell her that you are going to place your finger in their rectum. It's a good idea to talk her through each step as effective communication will prevent it from being painful and you want her to enjoy this.
4. Place your well lubricated index finger against her anus, but do not yet insert it into the rectum.
5. Ask her to bear down as if they are trying to have a bowel movement. This helps to relax their external sphincter and should decrease discomfort. As she bears down, gently push forward until you have placed the entire finger within the anal canal.
6. As your finger enters you may run into some stool, which should easily move out of the way. If this occurs, do not force your finger forward you should not meet any resistance. The only instance would be a mass in her rectum.
7. By gently rotating your hand, you can use the pad of your index finger to feel to the left, right and then directly backwards (i.e. towards yourself) and along the sacrum.
8. You can ask her to squeeze her rectum around your finger. This is performed to assess how well the muscles in your rectum and bowels.
9. That would conclude a female rectal exam. How it proceeds after is according to the both of you. As I have said before, take your time, make it pleasurable rather than painful if you would like to repeat it.
Please be aware that there may be a small amount of bleeding from her rectum, particularly if she has haemorrhoids.
Hope this helps.