A bottle with holes in it is probably not a highly original idea for a watering can substitute. Still, for those who may find it useful, this is the way I make mine.
Step 1: DRILL HOLES
As an alternative to drilling, you can heat up a nail in a flame and melt holes in the plastic. Hold the nail with pliers.
You also need to make a breather hole in the handle.
It is easier to make a too-small hole larger than to make a too-large one smaller. Also, you can always add holes. Taking them away is more difficult. Try the jug out occasionally as you go and adjust the hole size and number to suit your needs.
For larger plants and for faster water flow you might want larger holes. I also use these jugs when I work with cement, for adding small amounts of water to a bucket of cement.