Cut yourself a piece of tyre roughly about the size of the grip you want to make-
Its easier if you hold it to the handlebar to get an approximate size. When measuring make sure you measure the width properly- too wide and it won't grip the bar, not wide enough and it'll have an irritating gap.
Mark out and drill/ poke small holes at roughly a finger width apart along both sides of your grip sized bit of tyre- its much easier if you drill the holes, but make sure you put something behind it and keep fingers out of the way
You will now have a bit of old tyre with several holes in it- should look a bit like this...
Using a piece of scrap inner tube 'lace' the piece of tyre as you would a shoe- joining the two long edges together- I used an old punch to aid with the poking
It should begin to look like this...
Once finished, tie the ends of the inner tube off and fit to the bike. I've found that the tyre grip grips the handlebar better if you first wrap it with a small piece of inner tube. If you have any to hand- pop a old cork in each of the bar ends to finish it off.
As always I'm assuming a reasonable level of tool competance and dexterity, if you feel you may injure yourself in attempting to make the grips please don't as I accept no responsibility for any injuries resulting.