Yes, I had long time not going to confession and was out of practice, forgotten how!
Here's just one guideline: catholic.org/prayers/confession.php
Also, keep in mind, you don't normally have to get into specifics about each sin. You can say the category, number of times. If you don't remember, round off...5, 10...100s, thousands...for each mortal sin.
If the priest wants more detail, he'll ask. I used to do lengthy confessions. Now, I see they normally aren't necessary. I bring a paper with me, quickly go down the list. I go prepared so that I don't leave any out.
It's good to say a prayer before the examination of conscience that we be helped to see all our sins.
Then, I try to end that I am sorry for all these, and also any other sins I don't presently remember.
Now, after that, you may indeed forget some sins and recall them later. If these are mortal sins, you are forgiven for those, as well, assuming you are repentant, etc., but have an obligation to bring these other sins to the attention of the confessor next time you go to confession.
Oh, I heard one priest, think it was on Catholic Radio, once who was talking about he actually liked to see people confess who hadn't confessed in, say, 30 years! He called them "the big fish"!