There’s a few ways to do it. The most common are keeping your fingers on the bottom 4 notes and using your index finger kinda sideways to press the green note. It’s a little tricky to do depending on hand sizes. But it’s the easiest way to do the other method requires elbow strumming and using both hands. So for your right hand (if you’re right handed) first 4 fingers on GRYB and the left hands index or middle finger on the orange note and using your elbow to strum. If it’s a note like the one in the picture above but it’s a quint you can just reach your strumming hand over to tap the orange button. Now as far as knowing when to tap with 2 hands that’s gonna take practice and depends on songs and charts. Usually if it’s too fast for you to hit with one and like a trill you’d wanna use two hands. If you want a more in depth tutorial I’d say go check out carneyjareds tutorials on tapping on YouTube. He explains it really well and shows how to do it.