The first thing you need is something to throw at.  The best thing is the end of a log.  The bigger the log the better, because it is easier to hit.  A slice off the end of the log about one foot thick is perfect.  This is known as a hawk block.  If your block is not very big you should set it on something (another slice of log) so that it is off the ground.  It would be good if the center of your block is about waist high.  If you can't get a log, a heavy piece of lumber will do.  Remember, it is easiest to stick a tomahawk in the end grain but if you have to use a piece of lumber, set it up so the grain runs up & down. Whatever you get, be sure to set it up in a safe place and somewhere that if you miss and throw past it, you won't lose your tomahawk.

To begin, stand with your back to the block and step away about five or six steps.  Turn and face the block.  Hold the tomahawk by the end of the handle just as if you were going to chop wood. The most important thing to remember when throwing is not to flip the tomahawk. Throw it just as you would a baseball and it will turn by itself.  Throw it like you would if you were playing catch, not like pitching a fast ball.

The trick is to throw it the same every time.  If it doesn't stick move closer or farther away. From five or six steps away, the tomahawk should rotate one turn and stick in the block with the handle pointing down.  If the tomahawk hits the block before it makes one full turn, then move farther away.  If it turns more than one turn before it reaches the block, move closer.  Move only a little bit each time, it doesn't take much.  Put a marker on the ground so you know exactly where you are throwing from.  It won't take long and you will be standing in the same place and sticking it every time.  Then you can practice for accuracy.