Nipping Basics - Offset Tile Nippers
1.
Tile nippers do not cut tile, they crack it. The line of cracking will generally be a straight line across the tile in the direction of the jaws.
To get a straight across crack, place the jaws a at right angle to the edge.
To get an angled crack, place the jaws at an angle to the edge.
Only place the jaws over the tile approximately 1/4 inch (see the right way in figure A).
Placing more of the jaws over the tile will make it VERY difficult to cut
Be sure to hold the nippers with the flat side of the jaws facing away from your hand. The handles should be held as near the end of the hanldes as possible. This gives the most leverage. A gentle squeeze produces high pressure in the jaws.

ALWAYS wear safety goggles when nipping.
Until you are adept at holding the tile so that both pieces are still in your grip after the cut is made, (and all your fingertips are still attached to your hand) I suggest placing a towel over the nippers while cutting to keep the pieces from flying about.
You can also place your hand with the nippers holding the tile in a paper bag.
Nipping gets easier and more accurate with practice. Practice dividing tiles into four smaller squares by first halving one square then halve each remaining one.
Circular shapes can be achieved by nibbling away at the corners of your tile.

Nipping Basics - Wheeled Mosaic Cutters

Wheeled Mosaic Cutters are basically used the in the same way except that they will cut, not crack, the tile so you should place the cutter in the center of the tile where you want to cut.
If the nippers are not held at a right angle to the tile, a curved cut may result.

FANTASTIC NIPPING TIP
For harder to nip tiles such as Relics and other porcelain & stone or you have a lot of nipping to do try this tip. As a nipper-elbow sufferer this one worked wonders for me. I wish I had thought of it!
Using a 3 ½" C-clamp, clamp the bottom handle of your nippers (or glass cutters) to your table. Slide a 15 inch, 1" diameter PVC pipe over the top handle and nip away! For extra cushion, slide foam pipe insulation over the pipe. (It will resemble the old water pumps you see in Westerns) No squeezing, no pain!! You wont believe how easy it is! As an added bonus, nail a plastic bucket (or plastic jug) to the side of the table, under the nipper to catch the falling pieces. Cut out the front 3" lower than the back. Before nailing, position the bucket so that the pieces hit the back of the bucket, then fall in.
Safety Goggles are still strongly recommended.