This list is not exhaustive, and a lot of writing to prisoners is also about just learning to get comfortable making contact with folks you likely don’t know.
First off, the rules differ based on the prison. Obviously, material you send in and which is sent to you is censored. What you can get away with as far as writing, but also the length and physical contents of a letter (images, etc.) may differ with different prisons. If you want your letter to get through it is probably best to avoid a lot of ink, sparkles, glue, stickers, and so on and to keep the letters below 20 pages (this might seem like a lot, but it is sometimes nice to send prisoners photocopies of books or articles, poems, etc.). Sometimes it is good to write on one side of a piece of paper so that your correspondent can use the other side as paper costs money (as do stamps and envelopes) and prisoners are generally not swimming in cash-money. This is another reason to understand if you do not hear back from a prisoner, it could be that they can’t afford to write back, are busy with other things, or are just not interested.
Because your letters are being read it is important to self-censor (obviously within bounds). Keep in mind that what you write may have an inadvertent impact on the treatment of your correspondent by screws, etc. And obviously you don’t want to write anything that is going to get you or others into shit.
Obviously, censorship means the letter’s already been opened before your correspondent receives it. Number the pages (e.g. 1/12, 2/12, etc.) so they know how much you’ve sent on, and print your return address on the pages of the letter itself in case they do not receive the envelope and/ or have lost your address.
Sending in books, or other materials is also going to depend on the rules at whatever specific site. Once you are in contact with someone you can ask them what those rules are if you would like to send something in. Oftentimes books, for example, are expected to be sent from a distributor/ publisher but will still have extra rules around that and it is up to you to know those before sending material on. Once you find out the rules some radical publishers offer deals. AK Press, for example, will send books from their catalogue to prisoners at a %30 discount.
As others have emphasized, when you write to prisoners don’t make promises that you can’t keep.
Decide how much information to give about yourself by evaluating how much you know about the person you are writing to, but don’t be afraid to talk about your life. It’s boring inside, and a lot of people lose contact with friends and family outside so having that kind of lifeline can be important. Simultaneously, we feel it is important to note, that we do are not necessarily familiar with the prisoners listed on this site and exercising caution is sometimes a good idea at least until you feel you have established a mutual trust and comfort with a prisoner correspondent.
You may want to also check out other prisoner correspondence sites for more tips on writing to prisoners.