I'd suggest going to the MainMenu of the Write a Prisoner site, click Home and then down to the FAQ (frequently asked questions)- There is a lot of good information in the FAQ section of this website, which are good to go back to if you start writing to someone and have a more specific question pop up.
There are two main ways of messaging prisoners.
Snail (postal) mail and electronic messaging. On their profile there should be a line for "Institution email" which might say JPay or Corrlinks, or one of the others, Connect Network(?) and down further on the profile will be their prison mail address for snail mail.
If you're worried about using your home address or live with other people and worried about them finding your mail before you do, electronic messaging could be for you. You could also rent a P.O. Box, or use Jmail which gives your pals a mailing address in America and then they scan the letter and email it to you - this is for snail mail.
The Write A Prisoner “email” service is for intial contact only, and is meant to be a one off to make an introduction to a potential pen pal. They limit you to one a month so it's only useful for one inmate. You cannot use the WAP email for the same inmate twice, even a month apart. You can send another WAP email to a different inmate once a month has gone by since your first one. The message is sent as an electronic message to WAP, they print it off and send it via snail mail using the name and address you provided when you set up your account with WAP. If you want to write any others then you'll have to reach out yourself through one of the other ways of contact.
These letters are sent twice a month, once on the 7th and once on the 23rd of each month. These letters are notorious for being slower than regular mail that you send yourself and now with the Coronavirus pandemic going on, mail is even more unpredictable than ever. You could be waiting one week for a reply, to upwards of three months. Extra patience is needed now, but also patience is a really key virtue for all people who choose to write to prison pen pals to have, and work on.
JPay is one of the electronic messaging services available in some prisons, and I don't personally use it but there are many people on the forum that do, and a lot of threads on JPay advice. Basically you make an account on their website, search for your inmates name and add them as a contact. Then you buy virtual JPay stamps to write letters that way. You can usually include a pre-paid reply stamp for your pal to reply to you. But in Texas (and a few other States like Colorado, some prisons in California) you can JPay them, the prison will print it off as a message and your pal will snail mail you back. So if you write to an inmate in Texas or Colorado make sure to include a return address in the body of the message so they have a way to reply to you.
Corrlinks is for Federal inmates (prisons followed by USP & FCI etc.) and a few State prisons use it. Though I believe the Corrlinks is a different procedure for State Prisons. For a Federal inmate, you have to send the email you used to sign up to them via snail mail and they will add you on their end. Then you’ll be sent a code to enter into Corrlinks to accept the inmate.
You can sign up to Corrlinks for free, using an email address. You can sign into the website to read your mail/manage your account details. Or you can buy a premium account for around $6-$10, that's for a whole year and lets you put the app on your phone (which is handy cause you don’t need to sign in all the time, and are notified when there’s a new message). Writing and sending messages for us on the outside is free if you sign in to their website. Inmates however, pay by the minute to use Corrlinks, and can print off messages for a fee of around 15cents (I believe).
Corrlinks allows one email with a max of 12,750 characters and does not let you send photos, only writing.
Then there is good old snail mail which is my favourite way to correspond with inmates.
Basics of snail mail are: stick to black or blue pen on white lined/ruled paper, no tape, no glue, no stickers, no glitter, no white out (scribble or cross out any mistakes), no stamps, no money, basically nothing but paper in the envelope. Write your PPs full name and DoC number correctly on the envelope, as well as your full name and complete address as the return address.