Nintendo just shared on his Youtube Channel a tutorial video explaining step by step how to take screenshots ingame and share it on your social networks.
This one is rather simple, since there’s a “Capture” button dedicated to taking screenshots when you’re playing. It’s on the left side of your Joy-Con or Pro Controller. Once pressed, the system will confirm the screenshot taken and store it in your Album. If you want to share it, you’ll just have to go select it, put some text if you want to, then chose to share it on Facebook or Twitter and voila !