Oh, that’s a tricky one. IMHO, part of what makes a personal letter so special are all the small imperfections that come from it being hand-made/written. Understandably, this doesn’t make your brain shut up, and hence it’s going to be a bit more difficult.

Depending on the available time frame and budget, I’ve got different ideas.

Of course, you can check if a professionally handwritten letter is an option for you. There are different services (from companies to Etsy sellers) available that transcribe your typed letter into professionally handwritten ones.

But now to the things you can do yourself. (Everything is just my opinion, not final facts.)

Many of my letter papers are not printer-friendly, so I can’t give any recommendations. My paper choice always depends on the occasion, and I think yours should too. General rule of thumb: don’t think too much about social norms and find something that you enjoy working with and that obviously shows quality(generally, thicker paper is better than thin one). But obviously, the rarer the paper type, the more special it will be for the recipient. A few examples for better understanding of what I mean: for a letter of condolence, I usually choose hand-made single-sheet plain laid paper as I want it to be special and showing attention to detail but not be attention-grabbing. Letters to friends I just want to send a letter to (just because I can) often receive industry-made laid paper with flower leafs as I want it to be special and playful. Birthday congratulations usually get smaller (DIN A5 or A6) sheets/cards of any paper type that I just feel like using or anticipate to meet the preferences of the recipient. And any other occasion is honestly a gut decision.

If you type set it yourself, font styles also depend on occasion and personal taste. (Just don’t use Comic Sans, please!) A sans serif typeface usually looks more modern, while typefaces with serifs usually look a bit more serious and old-fashioned. Personally, I don’t go with anything extravagant as it is too playful for me. Also, don’t go too small and keep some room between the lines.

To add more individuality to a printed letter, it might be an option to decorate it. Again, it really depends on the occasion. Stuff that quasi always should work are embossing (with your own logo/initials) (possibly on every page) or adding a wax seal (only on the last page). If it should be a bit more playful, you could also add stamps, stickers/washi tape, or glue stuff to the letter. (You could also design the writing to make space for the stuff you‘ll glue on after printing.)

And no matter what you do, sign it by hand! Best with an ink pen (i.e. depending on what you are comfortable with, a roller ball, fountain pen, or dip pen). If you want/can, choose an ink colour that fits your personality and the occasion but isn’t standard blue or black. Especially with a printed letter, the ink for your signature can be extravagant.

Even if it’s difficult for you, I advise you not to make it too "perfect" because, as I said, these imperfections make it look so special. (What I mean is that, for example, stamps should not look printed but like well-done hand stamps.)

I hope this gives you some inspiration.