Teespring is simple, here's how it works: You can either create a design yourself or buy one from someone. You can get very good designs on fiverr.com for $5! Designs can only be 10 colors or less, and the less colors, the more profit per shirt.
Create your campaign. This involves choosing the shirt, price, setting your goal, and then making a page for your shirt. Example of a page: http://imgur.com/QtPkJRx
3 shirts must sell for them to be printed, shipped, and for you to be paid. Prices can vary from $12+. I have found that the best price is $21.95, you want around $10-$12 profit per shirt.
Market the shirt, you can pay for facebook ads, post on your own social media networks, pay for google ads, and other options. The best ads to use on facebook are Page Post Engagement and Website Conversions, I recommend paying $5 a day to test your shirt, if you do not have sales after the first couple days, kill the campaign.
Payment: US citizens can be paid via Paypal and Non-US citizens are now able to be paid via Payoneer. You are usually paid within a week.
Shipping: Shipping is cheapest for the US, if you plan on selling in the UK, you can sell on fabrily.com, just like Teespring but for the UK!(I believe they are merging soon!)
The best part of it all: Teespring handles all of the shipping and printing. You do not have to pay anything, they simply take those costs out of your sales. If you sell 50 shirts at $21.95, you will most likely get around $12/shirt.
If you have any questions comment below and I will try to answer them the best I can!
If you're interested in signing up: Referral link: https://teespring.com/invites/bk1mn1 (You and I will receive a .50 bonus for every shirt you sell!) Non-referral :www.teespring.com