Putting in a tap

I'm not good at drilling and DiY, so I don't fit taps, I just dunk my watering can in the top

It's easy enough to do, but don't make any holes till the tap arrives. It will probably be assembled - you just unscrew the front from the back. Drill your hole (probably half an inch diameter, but don't take my word for it - check). They never seem to seal properly, so use some silicone sealant when assembling.

Not sure what a self tapping-tap is. If it genuinely cuts its own thread as you screw it into the hole drilled in the butt, then fitting it should be a simple matter of drilling the correct size hole (for anything self tapping usualy a tad smaller than the diamater of the fitting) and screwing the tap into the hole.

If the tap is the standard water butt type tap then it is just a question of drilling the hole, fitting one washer over the threaded portion of the tap so that it is on the outside of the butt, inserting tap through hole and putting another washer on threaded portion of the tap (this time inside the butt) and screwing on the retaining nut to hold everything in place.

Use a 'spade bit' for drilling the holes. They come in a variety of sizes and can be bought individualy from good hardware shops.

If the tap is 'self-tapping' the drill size is very important. If just a standard butt type tap then hole size is not so critical (as long as it's wide enough) as the washers will keep things water tight.

Hope this is of some help.

Thanks guys for all your replies. ( Nice to see you Snuffer, I had forgotten though...) I was hoping to be able to leave the lid as is with just two bung holes, to save problems with wind, debris and drowned bodies. (I spend a lot of time being RNLI to beasties in the current water tub.) So I'm hoping I don't have to put a retaining nut etc inside; is there a way of doing that using a waldo of some sort ? (I'm thinking of a long stick with the nut on it, but how do I attach it ?) And I know how I can cut the lid, but is there a particularly good way of cutting/attaching it so that it stays on ? (Would a hot air paintstripper soften the lips enough to just lift it off ?) This place is a bit wind blasted.
Sorry to be all needy and demanding, but I believe firmly in learning from other people's trials and errors in order to minimise mine.
I already know to take my drill bits with me when I go to the shop today, so that I know whether to buy a new one or not, and I have checked my sealant - so much appreciated !

For all my taps that I've put in, I have used a piece of 22mm copper tubing
Heat it up with a blow torch/gas hob and then press at the area you want.
Makes a perfect circle, just remember to clean the inside of the hole (push from out to in) as there will be a couple of tiny little bits that may cause the washers to not seal perfectly.
The size of hole isnt critical (clearly its got to be close enough) as the washers seal any gaps.