There's no mnemonic - it's a question of whether the French verb needs à. Envoyer and dire do - envoyer quelque chose à qqun and dire quelque chose à qqun, so your first two are wrong. The correct grammar is

Je leur ai envoyé...

Je leur ai dit...

But the causative doesn't need a preposition: faire qqun faire qqqchose so Je les avais fait... is correct.

There's no mnemonic - it's a question of whether the French verb needs à. Envoyer and dire do - envoyer quelque chose à qqun and dire quelque chose à qqun, so your first two are wrong. The correct grammar is

Je leur ai envoyé...

Je leur ai dit...

But the causative doesn't take à unless there's also a direct object. So je leur ai fait comprendre la phrase but je les ai fait comprendre.