Ahh, satcheling tanks, my favourite pasttime.
1.) Approach the tank from behind.
2.) You're not close enough.
3.) Stand behind the tank, and as said above, aim 45 degrees up.
4.) Throw the satchel charge so that it lands on the engine deck. It is flat so the charge will settle there, and it also has the weakest armour. If you're close enough the charge may even land on the turret roof. That's OK, it's flat and weakly armoured too.
5.) Turn around and run like hell for cover.
6.) ? ? ? ?
7.) PROFIT!
90% of the time, this kills the target outright. "Heavies" like Matilda/Valentine might survive, but even then, they will be burning and just a scratch from annihilation. Noob tankers will bail out and try to fix their tank WITHOUT driving to safety and/or checking their surroundings, so you might still get the driver this way. And if you're not spotted, you usually have a second satchel to throw.
If a light tank is too close to its buddy, you might get lucky and catch it in the blast wave too. Once on Bardia, I threw the Ladung on top of a Matilda; a Cruiser spotted me and overran me. However, it drove right next to the Matilda in in the process, so the when the charge blew up both tanks went up in flames.