If you've added Next Keyboard to your devices keyboard list, you simply have to switch the stock iOS keyboard to Next Keyboard.
Go through the following steps to bring up Next:
1. Open your favourite messaging app
2. Bring up the keyboard by tapping into a text-box
3. Tap and hold the globe key located in the bottom left of the keyboard
4. Slide your finger up to Next Keyboard
You should now see and be able to use Next Keyboard.