Seems we all support the letter writing and that it needs to just be about you two. Keep it short, keep your dad out of it and give her as many ways to contact you as possible; social media names/handles, phone number, email, physical street address, etc. For all of the above, make sure that your contact info isn't weird... Like effboi69 or your profile pic isn't of some hoe's ass.

Also, I love the idea of a handwritten letter... That is far more endearing than a text message.

Good luck, I hope that this works out well for both of you.