What you'd do is that you'd write a distributed remailer where fixed-size messages are sent on fixed timeslots, possibly with some noise in when it's transmitted, with a message always being sent on its timeslot, even if a dummy message must be sent.

I've been writing a system like this in Erlang, intended to be short enough that you can take a picture of the source code and then type it in by hand in a reasonable amount of time, as a sort of protest against Chat Control. I'm not sure I'm going to release it-- after all, they haven't passed it yet, and there are all sorts of problems that this thing could needlessly accelerate, but I've started fiddling with it more intensively recently.

Don’t things like Freenet do similar?

Except that every user is also a node, thereby mixing their personal traffic into a share of network traffic. Or so I understand it.