Translation:We used to donate to various charities.
Before the noun, "différentes" means "several", "multiple" or "various". After the noun, it means specifically "different" as in "not the same".
So, "Nous donnions à différentes associations." would mean "we give to several charities."
And "Nous donnions aux associations différentes." Would be used in a case like: "My husband and I are both philanthropists, but we give to different charities."
Although, as with any language, there are exceptions and people don't always use the word placement this way. It all depends on context.
"Charities" usually are "des associations" but not all "les associations" are charities. Think of the NRA in the US or "l'Association des Amateurs de l'Authentique Andouille Auvergnate" in France, which is a club of lovers of the authentic "andouille" (small sausage made of chitterlings) from Auvergne.
"We donated" could translate to "Nous donnions" depending on context.
The imparfait tense is used to describe ongoing activities and states in the past. So, for example:
"When I was young, we donated a lot of money to help the poor"
would translate with the imparfait because the action of giving was ongoing:
-->"Quand j'etais jeune, nous donnions beaucoup d'argent aux pauvres."
But, if the act of giving was a single time action, it would be Passé Composé:
"I gave you 5 euros last week!"--> "Je vous ai donné 5 euros la semaine derniere!"
Although, "faire donner" or "faire un don" is a closer translation of "to donate". "Donner" by itself can safely be translated to "to give" in most situations. It's just like in English: "We donate our time" and "We give our time" are synonymous.
"Différentes associations" means "various associations". With this meaning, there is no singular.
With the meaning of "different associations", as the plural of "a different association", the French would be "des associations différentes", as the plural of "une association différente".