Hi! Pirrip here! I've written up a little guide to finding a company to printing your doujinshi, so you can take them to conventions and sell them. You can find my original post on
my tumblr, but I'll just go ahead and re-post it right here:

I'm not an expert, but maybe my experience can help you. Masao and I have begun to draw doujinshi, so we can print them to sell at anime conventions. Doujinshi literally means "Self Made". Whether it be fan or original art, if you make it yourself without a publishing company, it's doujinshi. Drawing doujinshi is really great if you want to be a professional comic artist or mangaka(manga artist) one day. It gives you the opportunity to get some serious practice in drawing or producing work before you're ready to embark on bigger projects. And like many doujinka(doujinshi artists), you may want to pay homage to a series you admire.
Assuming that you've already put in a disgusting amount of time finishing all of your pages--whether it be on paper or drawn digitally--you will need to resize your pages to fit a standard size, or decide to print your book in a custom size. This gets a little complicated because printing companies do not always have the size you will want to print without charging you extra for a custom size. So with that in mind, I will share with you the companies I use to print, and what sizes they offer:
KA-BLAM (in Orlando)
- Standard Size: 6.75" x 10.25"
- Manga Size: 5" x 7.5"
- Magazine Size: 8" x 10.5"
- Custom Sizes: Yes
- Adult Content: No
- Minimum Order: 1
SIPS Comics (in British Colombia)
- Standard Size: 6.625" x 10.25"
- Custom Sizes: Yes
- Adult Content: Yes
- Minimum Order: 100
There are other printing companies out there, but these are the ones I use. Ka-Blam is great because they offer more standard book sizes and don't require you to order more than you need if you think you'll need less than 100, which could save you a bundle! And the quality is really very good. Make sure that you are ready to upload all of your files to them digitally at least one to one and a half months in advance of when you need them. You'll need to upload your images as TIFF files at 300dpi. Ka-Blam will also print posters, hardback books, and other things. You can visit Ka-Blam for more details.
Sips is great because aside from printing very high quality books, they will allow adult content. They are also extremely personable, and willing to answer any questions or address any of your concerns very promptly through email. If you are interested in printing explicit Yaoi specifically, there are very, very few printers in the U.S. that will allow printing this kind of content. Sips is in Canada, and although I live in the U.S. they were able to mail my order back to me in less than a month. They also offer reduced prices if you're planning on printing a lot of books. Sips can print PDF and TIFF files between 250dpi and 600dpi(really high quality). You can visit Sips Comics for more details.
Both companies will let you print books in full colour or black and white(greyscale), but keep in mind that colour will be more expensive to print than black and white. As well, both companies allow for page-bleed--so your frames or drawings can go all the way to the edge of the paper.
WHATEVER YOU DO do not print with FedEx Office/FedEx Kinkos. You might be able to find someone who works there willing to help you put together your book, but there are a number of drawbacks to this. It's relatively expensive. They are not a comic printing company, so they do not know what paper would work the best--so you'd have to know what paper weight and texture you need up front. The quality is less than optimal. Cutting the paper to your planned size is a very dangerous endevour--they literally cut it with a machete-style paper cutter. If you are planning for the page to end at a specific place, good luck. Something might get cut off in a weird place.
If you draw seriously and want to start making doujinshi, Doujinshi Edu on deviantart is a great resource to see what other artists are doing on their doujins. You could browse through the galleries there and meet other artists. And most are happy to answer any questions you might have. As well, you can ask me anything here on tumblr.
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My doujinshi circle is called Corgi Nub, you can find it on Facebook. We are Pirrip and Masao on deviantart. And we're planning two new doujinshi collaborations with Shadi and KawaiiPV.
Here are some shots of our books if you're curious(the printing looks amazing in real life).

This is from our Axis Powers Hetalia fanbook printed by Ka-Blam. It features Germany, Italy, Poland, and Lithuania.

This is from our Gundam Wing +18 doujinshi printed by SIPS. It features mecha fighting AND yaoi.
Our Loveless Doujinshi was our first doujinshi(it's the white cover with the pink butterfly design at the top)... and printed by FedEx Kinkos... And the quality isn't that great, so I'm not even going to upload a preview. We may reprint it in the future.