First I made a list of everyone I wanted to write during the month. (Unlike many who take part in the challenge and find new pen-pals, I write only people I know and use it as a chance to keep in touch and/or catch up). I divided it between those I want to write weekly, those I want to write before the 14th (mainly sending Valentines cards, for instance to great aunts), and those I want to write after the 14th.
Then I printed off a master address list and seals for letters. The LetterMo calendar I printed off then took the names from my wanting to write to list and scheduled out a plan so everyone should get their cards in time, but spread out the writing for me. I also gathered some stickers Valentines stickers and stamps to have handy.
I discovered that the cards I was planning to send for Valentines Day had square envelopes; which requires extra postage and the upcoming pretty tulip 2oz stamps don't come out until the 14th... Thankfully I have a stash of extra envelopes I've picked up over the years and was able to find rectangular envelopes that they fit in well and would qualify for Forever stamps, which as you saw from the first photo, I have a nice set of heart stamps that will work great!
I got my postcard set and put some postcard stamps with it. That way if the plan doesn't work for some day, I can just pull out a postcard, write/address it, and stick a stamp on real quick.
Then I put everything in this pretty expanding file. Front has the items in the first photo, back slot the postcards. And in the middle each slot has like 3 days worth of cards for the first two weeks, and each of the next weeks has their own slot it will be split up as it gets closer.
So, are you doing LetterMo?