While playing the game, press Home and go to Miiverse. Then choose the game's community and post. You can choose between top and buttom sceen. You'll share the screenshot at the moment, when you pressed Home.

But not every game will let you go to Miiverse on OG 3DS/2DS because this game may use extended memory mode like Mario Maker, Monster Hunter series, Pokemon etc.

thanks so much il try it now!

OH YES thanks so much it worked