As someone with a fair bit of experience (both top and bottom) I will say that LEVI's little guide that he posted is pretty well correct. There are a few things that I would like to point out, as they may help you to be able to get going on your journey to fisting a little nicer.
FIRST: I would like to add that there is a little more to the art of getting introduced to fisting than taking the process slowly increasing by one finger at a time. As you're beginning to finger (especially as you begin to increase the number of fingers), it can become quite helpful to twist your hand a bit. For some people, twisting as you go in and/or out is a better fit for them, but for others twisting while it's in and not going in or out is a little more pleasurable. This will take a little getting used to and a little getting to know your partner. When doing this, it is actually better to use the latex free gloves, mainly because they do not deteriorate when they are introduced to Crisco.
NEXT: When working your way up in the number of fingers, it is very important to remember that most beginners will start to have a certain level of difficulty once the knuckles begin to to be introduced, mainly because that begins to reach the widest point of the hand. At this stage, twisting to a certain degree will become more critical, especially since it will help to keep your lube more evenly applied. Also, it is important to keep in mind that when you are at 4 fingers, introducing the thumb is going to become the next road block because now you're adding to the width of what is being entered. To help work with that, this is the most critical stage to begin going back down in the number of fingers, then exploring your bottom. Generally speaking, the prostate will bring much pleasure, but every bottom has their different spots that get them purring. As you work back up to 4 fingers, you should begin to get to know these spots. This will become key for the next stage.
LASTLY: Once you get to the point of getting 4 fingers in as far as the knuckle, make sure to start exploring your partner to see what spots make them loosen up, and which spots make them clench. It will also be good to get to know whether fingers curled up or fingers straight out are better for your partner as you continue to push. It is also very important to make sure that you leave your thumb sticking out so that you cannot push the knuckles too far in right away. At this stage, most bottoms will find it less uncomfortable to have the knuckles running in line with the ass crack, instead of going against it. Once you begin to get loose enough at this comfort level, then it is time to begin to introduce the thumb. It is not at all uncommon for the knuckles to not quite go as far in on the first couple of tries once the thumb is introduced. Do not worry if this happens. It will pass.
ABOVE ALL ELSE: When you're beginning to go deeper into your partner, as a beginner, you will notice spots where you will experience some pain at first. It is very important that when you reach this spot where the pain becomes a little much, not only must you stop pushing in more, but you should also ease off the pressure just a little bit to take the edge off the pain. When you experience this pain as the bottom, it is important to have some sort of signal to your partner to pull a bit of pressure off. When this happens, avoid your urge to pull out. Instead, focus on taking deep breaths and waiting it out. Once you have started to relax and this initial over-whelming feeling starts to pass, it is important for the top to begin to explore a little with their fingers to help encourage the muscles to relax. It may also be helpful at this point for the bottom to clench their abdomen and push like they were trying to push something out. This will help to open the muscles and let them loosen up to get ready for the next level. Once they have loosened up and relaxed a little with this, THEN it is time to start to push a little more. Always remember that when it comes to fisting as a newbie, slower is better. It will take some time at first.