This is what I think james would have written to Harry in a letter. I might do more letters from different people to different people im not sure yet. please check and tell me what you think!

disclaimer: I obviously don't own harry potter or any of its characters.

Dear Harry
How's my little man ?
Its kind of weird writing this letter for your 11th birthday when you're a few months old and sitting in my lap!
And your mother is not making me write this letter (wink).

I want you to know that you and your mom are the best things in my life. I still remember the day I fell in love with her. I love her red curly hair even when she complains about it. i love her emerald-green eyes which you inherited. I love the sound of her voice even when she gets mad. I remember the day clearly when she said she'd marry me. I never felt so much joy till the day we found out we were going to be having you.
I honestly hope you never have to read this letter but with this war going on I also hoped you wouldn't have to grow up in this.
If, by some chance some unfortunate chance something were to happen to me or (hopefully not) your mom, I want you to know I am so proud of you. I know the world may seem like a nightmare with no chance of changing but I'm sure you will see the sun after the clouds have gone.
Okay, enough with the heavy. If this is going to be the one and only letter of mine your going to get I don't want it all bad and your mother is glaring at me and my inner marauder is basically kicking me for writing that (it's a thing! Trust me!).
Just one last thing if padfoot becomes your guardian I give you my permission to kick his a..I mean treat him with respect (lily saw what I was writing and if padfoot is reading this I will come back to haunt you if you think I'm whipped!).
Okay back to Harry. If I have...passed on you will probably only know stuff about me others have told you. I'm going to be honest when I was in Hogwarts I was... In your mothers words a 'big headed git' (toning it down for you) and be that as it may I know I was also honest (only lied when necessary and when you get to know Remus you'll understand), loyal (again when you get to know Remus you will understand what friendship meant to me) and I was hilarious ! I always made people laugh ! Even when your mom supposedly 'hated' me, I caught her smiling at some of my jokes.
You will probably hear a lot about the marauders and will know us as Hogwarts best troublemakers! You know us as prongs, padfoot, moony and wormtail.
When Sirius found out we were expecting you he started calling you prongslet, officially making you an honorary marauder ! (Okay lily is still reading this and is not happy I told you that but hey, your my son too!)
With that said (well wrote) I also know you are going to be a fantastic quidditch player ! I was a chaser on the gryffindor quidditch team and I know you inherited my skills. I remember during your first birthday padfoot got you a broom that lifted one foot off the ground and you nearly killed our cat and 'accidentally' broke a vase your mothers bad taste sister gave us. I'm sure if you hadn't broken it I would eventually but I was still proud of you!
Ok lily is getting mad that I'm taking ages to write this but if this is going to be the only letter you have of me I want it to be long ! Hopefully you inherited her ability to read very long letters.
The other reason she made me write this letter is to give you advice. Father to son advice...whatever that means.
Here goes my list of advice you should live by (just kidding but you should consider)
•be loyal to your friends (and fans).
•Only duel if necessary.
•Annoy sirius as much as possible (I recommend this very much, and if padfoot is reading this that's what you get for laughing at me for thinking I was whipped!).
•Listen to your teachers.
•Don't bully students even if they deserve it.
•Don't be afraid to try out for things (*cough*quidditch*cough*).
•Don't spend all your money at honeydukes in hogsmead (trust me).
•Do not think if you wake up an hour early you can fix your hair...just don't. It'll put you in a bad mood and anno y the hell (sorry for the language) you that you didn't do anything.
•If you are going to flirt with a girl (or guy I'm not gonna judge!) don't act like a jerk and pick on his/her best friend. -Hogwarts days.
•If you do find love at an early age like me don't constantly ask him/her out, pick on her, duel anyone they go out with or pretend to be dying. Does not end well...
•If I am gone and you are living with your mom, try to be there for her as I would have. I love you both so much that I don't know what I'd do without either of you.
•If you become quidditch captain make sure you get as much sleep as possible ! You would not believe how tired I was staying up all night to plan strategies! Eventually I started sleep walking and I ended up in the hospital wing as I fell down the stairs (if padfoot is reading this still = shut up)
•Also if you become head boy do not leave all the duties to the head girl. Not only will she be mad but she will be stressed as well. Also if you have any night plans (such as, running in the forest with your friends (you will understand when you get to know Remus))
Well that's all the advice I can think of. I'm sure you'll figure the rest out or your friends will help you figure it all out.
I will leave you with this, if something unfortunate was to happen to me.
I will always be proud of you. I love you so much that I couldn't bear to leave you. I know of the pressure that has been placed on your tiny (well probably not tiny now) shoulders but I know you'll come out the other end. If something does happen to your mother and I, I want you to know we will always be there with you. We will always be proud of you. And most importantly, we will always love you no matter what.
And if somehow you see how bad I was in Hogwarts or if someone tells you how much of a jerk I was, I hope that whatever memory of me you have is not tarnished. If there was any way I could go back and change the past...well I wouldn't. My past is what defines me. It made me the man I am today. But I hope you won't be mad at me for it. We all do stupid things when we're young.

Love always
James potter
A.K.A prongs

P.s. If Sirius and Remus raise you, you can trust them with anything. They were my best friends. Peter was too, but I think there's something wrong with him at the moment. He seems a bit withdrawn from us. Hopefully he's back to himself soon.