You good Sir are probably the first of many similar victims of the nanny state, increasing urbanisation, stranger danger, aspirational lifestyles.
Large families (No real effective birth control before the mid 60s'), big back yards were ideal situation to perfect rugby tackling skills. The lower down the pecking order the child was, the better tackler they had to be. Up against your bigger brothers and all their mates (the neighbours also didn't have birth control either), tackling skills were developed organically as per the Charles Darwin model evolutionary development.
Schools had playgrounds that kids could actually play footy on unsurpervised, frequently about 45 aside footy as well. There were no OH&S nazis were lurking behind every bush with their risk registers sucking the fun out of experiential learning. You learned to avoid doing things that hurt by working out that by learning what "hurt" felt like, not by reading about what "hurt" was in a book, or by having the aforesaid killjoy nazi just say that it would hurt and banning the activity for your own good.
There was time and space in back yards and local parks and schoolgrounds at lunchtimes to play footy and perfect footy skills. 40 aside Bullrush/British Bulldog/Bulldog/Scrag was played and the Teachers didn't mind. Parents didn't mind if Johnny was away from dawn to dusk at the local park, or in a variety of neighboiurs back yards, doing unstructured and unsupervised activities. They were confident that the community would look after its own, and kids were given time to be kids.
Jim was your mate James, not some place you went to to look at yourself in mirrors while you hump iron. Gym was a place where the nimble little buggers went to to practice Gymnastics. The waits in the Gym were limited to lining up behing the kid in front while they used the finished their routing on the mat or the vaulting horse - no other weights there. You walked. ran or rode your bike everywhere ... because you could and it was safe. There were a lot less vehicles on the roads to "eat" kids. You got fit in the same way that Kenya produces great long distance runners, not by following some S&C programme from a book. You just did it. Did I mention that it was always warm, never rained, and we never wore shoes, helmets or seatbelts in the car either.
None of this rant will help the present situation facing @EatSleepDrinkRuck. Clearly you weren't the youngest brother in an large family from quiet outer surburban or rural location surrounded by other large families of boys, attending a school that had a couple of hectares of land as a playground, or was located next to a (non dog walking) council park.
As ILTW and others have said, you have to replicate that rose coloured view of the "olden days" by just getting a couple of mates, some cones, crash pads and just doing it until you can do it no more, then do it some more. People get good at things by doing things. Until your muscles and reflexes are instinctively doing it, keep practicing it. The reason why some people are good at things is that they have done it more. It ain't luck.
It has been stated many times by many athletes somewhat ironically when some inane journo hack observes that they were lucky to <insert one of the following: win, make the put, shoot the basket, score the try, catch the wave, etc> that the more they practice, the luckier they get.
If you want to get better at something, there are no shortcuts, no magic formula in a book or on youtube. You can't buy it in a pill at the chemist. The Tooth Fairy can't help, and if you are of that ilk the Supreme Deity is not interested in helping you either. You learn through sweat and time.
There are plenty of tackling drills on the ARU Coachiing Resource website, and available via Google. Most work. You just have to get doen to the park with your mates and do them again and again and again.
Good Luck, and remember that in the modern game, knocking the smarmy prick with the ball off his feet isn't enough.
The job isn't done and the tackle doesn't finish until you are on your feet contesting for the ball, having released the tackled player as you get to your feet.
The last sentence is what separates a good tackler from a rider of the pine bench.
