It is possible to make your own jets up, but this requires a fair amount of work. By far the easiest way is to use some jets from another engine, this way a lot of the work is already done.
Ford Zetec 2 litre engines contain undercrown cooling jets, so these can be purchased from a Ford dealer.The actual jets cost about £7 each, so for that price are a bargain !
Installation is fairly simple, and most home engine builders should be able to manage the job.

This picture shows a block fitted with the oil jets taken from a Ford Zetec engine.

Undercrown oil cooling jets from a Ford Zetec engine, installed in the Turbo block. These jets are tapped into the main oil gallery and spray a jet of oil under the piston crown to help cool it.

To fit the jets the area on the block around where the jets are mounted must be smoothed flat {see top picture}. A small multi tool like a Dremel is fine for this.
Then an 8mm hole needs to be drilled 12mm deep. Make sure the hole is square, so the jet will sit evenly when installed.
A 3mm hole is then drilled inside the 8mm hole at the base and at an angle into the main oil gallery. This is the supply to the jet.
Fit the jet, bending the mounting as required so it sits flat on the block face. Mark the block where the mounting hole needs to be. Drill and tap the hole. The drill required is 4.2mm and the thread is M5.
Add a little locktight to the threads of the mounting bolts to be on the safe side, and add some sealant around the oil hole.
You may find, depending what pistons you use and where you position the jets, that they catch the piston skirt. In this case you will need to remove a small area of the piston to allow clearence. Be sure to remove an equal amount from each piston to aid balance.

Don't forget to thoroughly clean the block to remove all metal filings !!

TIP - You may find the holes in the Zetec jets a bit on the large side, infact they measure some 2.4mm or so. Crimping the ends slightly will help with oil pressure.

This picture shows some "homemade" oil jets.

The hole in these jets is 0.8mm, which should be adequate and help retain good oil pressure.

The jets fitted in the block.