reply to u/GrandRevolutionary99 at https://reddit.com/r/stationery/comments/1nrkuqf/i_need_help_to_create_my_own_letterhead_for_my/
Typewriter enthusiasts often use 100% cotton or high linen content papers with weights in the 32 pound range for 8.5x11. This gives you some nice tactile feel, but will also feed into most typewriters, even with a solid backing sheet. If you want to do thicker card stocks, then you might opt for a bigger standard typewriter which generally have larger diameter platens and more easily handle much thicker paper (they were meant for doing carbon packs up up to 10 sheets or more.)
When it comes to the look of your letters, you can generally choose between silk (clean, crisp imprints), nylon (almost as clean as silk, but with more "grain"), and cotton typewriter ribbon (which leaves a very grainy/old timey and "typewriter-y" imprint). Comparisons here.
I've got a small fleet of typewriters and prefer to use the pica sizes for personal correspondence. I also tend toward the cursive or Vogue typefaces for those as well.
In the US, a lot of stationers have pre-cut paper and envelopes for 6-3/8" x 8-1/2" paper which is a good size sheet for quick notes. My typewriter pen pal Tom Hanks' most recent letter to me was on a custom page of 7.125 x 10.25" and had space for design at the top and bottom with some reasonable space in the middle. If you do custom designs, be sure to order a box or two of plain stock to use as second, third, etc. pages behind your first page if you tend to write over your first page.
Naturally custom designing your own can be fun as well, but get a few samples of the size and weight you want and try them out before ordering in quantity.
Lenore Fenton can give you tips on making carbon copies of your letters if you want to keep them for your own files while sending out the originals: https://www.youtube.com/watch?v=JUJfCfqgsX0
Searching r/typewriters for stationery, letterhead, paper, etc. might give you some ideas as well.
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page from Wilf Beeching's Century of the Typewriter (1974 edition)
