Removing a grip from the handlebar is required to install the throttle and/or brake levers.
One method that uses common household items is described here.
First, pry the end plug out of the handlebar.
Slide a zip tie under the grip to create a gap, then flood the gap with rubbing alcohol. This lubricates the grip which then can be twisted and slid off the end of the handlebar.
Once the throttle / brake levers are installed on the bar, you can use a similar method to re-install the grips. Flood the bar and the inside of the grip with alcohol and slide it into place.
SAFETY NOTE: Do make sure the grip has dried in position before riding.