I want to get some new grips
Go to any bike shop and you can find a good pair of replacement grips for $6-12. You should be able to find some that are more comfortable than the stock ones. eBay usually has a number of dealers selling grips, too. The Pro Grips brand is quite popular among club members, as are Oury Grips. The Pro Grips have a relatively smooth surface, as opposed to the individual pads offered by the Ourys (similar to the waffle sole on Nikes). Another option is Grip Puppies, which are foam pads that slip over standard grips.
For grips that help smooth out vibration, see Reducing handlebar and footpeg vibration.
Removing bar ends: DON'T. A lot of members go through lots of work drilling, hammering, even using dynamite to remove the bar ends in order to remove the OEM grips. There's really no need, unless you plan to keep those old grips (but why?). You can easily cut them off carefully with a utility knife, then slip the new grips on with soapy water or hairspray. Another reason to cut them off is that some are glued on. There have been reports of one side being glued on while the other wasn't.
To remove: Take a sharp utility knife and cut off the old grips from the bar up. That is, do not cut down into the grip; work the edge of the blade under the grip and cut in an upward motion, to avoid scratching the bars or damaging the throttle tube.
Then take your new grips and apply a little soapy water to the inside of the grip and to the bar itself. You can also use hairspray, should you have some Extra Super Hold left over from the 80's. The soapy water or hair spray will make it easy to slide the new grips on, then after a few hours will dry to leave a good seal that will keep those grips stuck on there. Some members have reported slippage using hair spray, although most haven't. If you have some spray adhesive around, that would be a good alternative. Before you affix them permanently, see the first Note below.
Replacing the grips
There are several ways to do this job; here's one of them. It works.
This process is being shown an a Yamaha Maxim. Yes, some of our members do have more than one bike. The main difference between the two is that the Ninja has bar ends, but the procedure is the same. We will repeat:
As you can see, the old grips were starting to split on the ends, and were feeling loose.
Since you probably won't be trying to save the old grips, just cut them off.
Be careful when cutting off the grip on the throttle side, as the throttle tube is plastic. In this picture you can see a line where a previous owner started to cut into the tube. Luckily, it's not deep enough to be a concern.
Here are the new grips. Pro-grip 719's. The owner already had a set on his Ninja and liked them. They cost $13.50 at a local Honda dealer. You don't have to buy everything on the internet.
To aid in installing the grips, use hairspray or spray adhesive. This not only holds the grips in place once dry, it also keeps you from having to remove the bar ends. The hairspray allows the grips to slide over the bar ends on the Ninja.
Before you install the grips, make sure you identify which is the left and which is the right side. As you can see in the following picture, the grip on the right is larger in diameter. This is the grip for the right side. It needs to be bigger for the throttle tube.
The trick is to spray your adhesive into the inside of the grip to help it slide on. Once the hairspray dries it becomes tacky, helping to hold the grip in place.
While the hairspray is still wet, slide the grip over the end of the bar. Do one side at a time, so the 'glue' doesn't have time to dry out.
On the right side, make sure to leave a gap at the end, so the grip doesn't make your throttle stick.
Repeat the process for the left side. Make sure you let the hairspray/glue dry before you ride the bike. On a hot, sunny day this should be about 1/2 an hour or so. Also, make sure the throttle doesn't bind before you ride.
Looking good! Feeling good!