You can learn to design printables with free options like Canva, or Paint. These programs are fine for basic designs, but can be very limiting. If you would like completely custom designs, programs like Adobe Photoshop and Illustrator are excellent.
Wording is important when you're writing a letter, especially if you're trying to carefully word a request. If you're looking for financial relief or forgiveness, I have new hardship letters and apology letters. If you simply want to thank someone, there are plenty of new thank you letters.