One of the easiest things in the world.
To make a screenshot on the BLU Vivo XL3 Plus  just have to:
1. Go to the screen or image you want to capture on your BLU Vivo XL3 Plus
2. Simultaneously press the power button and volume down.

This screenshot is saved to your gallery in the "Screenshot" folder or "Screenshots"
3. Once there, you can tab to share, send the screenshot via email, facebook, whatsapp, or option you prefer.