In one voice, in writing, and against the briefing rather than against your taste. Those three rules solve most of it, and they cost nothing.
One voice
Collect your colleagues' views and turn them into one document in which the contradictions are already resolved. Five people each emailing the agency separately is not feedback but a problem you are passing on: the agency then has to decide who is right, and it cannot.
That also means: one person sends the feedback. Who that is may change per project, but not per round.
In writing
Verbal feedback lands differently with three people and cannot be found a week later. Write it down, even if you discuss it afterwards. Half a page is enough.
Against the briefing
The usable form is: "the briefing asked for X, this proposal does Y". The unusable form is: "I do not like it". The second is a feeling with no handle, and the agency can only respond by guessing.
If something feels wrong but you cannot say why, say exactly that, and look for it together. That is more honest than inventing a reason that then gets fixed while the feeling stays.
Four practical rules
- Give feedback per round, not piece by piece. Everything you send today belongs to the same round.
- Do not change direction without saying so. If you do, name it explicitly: "we are going somewhere else, and that is new."
- Say what is good. Not out of politeness: without it the agency does not know what to keep in the next round.
- Attach a decision. Feedback without a decision produces a next round that asks for a decision again.
And ask the reverse question too
"What do you need from us to deliver better work?" That question is almost never asked. In IKAg's measurements, brands score lowest with their agency on exactly the things they themselves control: briefing on time, steering and deciding.
Feedback against the briefing only works when that briefing is sharp itself. IKAg's briefing analysis walks through it free of charge.
See also: why the collaboration is grinding and how to write a good briefing.