Tackle:
For a tackle in which you just want to bring the opponent down i personally try to line them up as soon as i can and either hit the waist in which case ill start pumping the legs and try and lift one of their legs, or if they try to step try to get a leg and wrap the arms around, work on any size and believe me being only 5'9ish it works on all sizes.
if you've got a couple of team mates near enough incase this misses or the situation calls for it, if the opponent carrys the ball at chest height try and hit hard with the shoulder into the chest or somewhere in the upper body obviously keeping it legal and drive, if possible aim for the ball.
for side on tackles just focus on their legs and try and slide around them after the hit to get onto your feet and try and rip the ball.
Kick: for a box kick just pick it up take a step back if you have a good guard at the ruck and if going for height and are right footed aim for the right of the bottom of the ball and just put as much as you need into it, works well for me, for lengths just hit further up.
for a simple end over end just have a hand about half way up and strike the ball at the bottom.
Pass:
normal pass i just pull back and release the ball in the direction i want it to go in.
a spin pass, hand at the top and hand at the bottom of the ball, turn the ball on its side and pull back swing arms in direction of target pull with bottom hand, push with top hand and should work fine,
probaly better advice already given but thought i might as well help