This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
Your grip applicator should look like an elongated cone
about 12" - 15" long. Place the grip over the cone and
progressively work it over the cone until it is rolled on
itself like a donut. Place the large end of the cone on
the end of the bat handle and roll the rolled up grip past
where the cone touches the bat and now your grip is on the
bat but stilled rolled up. Roll it down the bat handle
without unrolling the donut until the donut shaped grip is
near the blade junction and a bit is just spreading over the
spread of the splice to the blade. Now unroll the grip
progressively back to the top of the handle.
At this point, your grip will be on the handle with the top
end extended over the top of the handle (rather like a foreskin).
Don't trim it yet. Holding the bat handle in both hands with the
blade down, tamp the end of the bat blade into a surface like a
carpet or firm grass so you distribute the grip evenly over the
handle. Now, you can trim off the foreskin of rubber to the edge
of the handle.
If you don't get it right the first time, just roll the grip back
down to the blade into a donut again, re-position and roll it back.
You can do this over and over until you get it just right.