The easiest way is to do it is with 1939 technology. Just the way they did it in The Wizard of OZ. Put the person in sepia clothes and paint them with sepia make-up. But if your foolish enough to shoot this without doing the research. You'll have to draw a mask around them and find a RGB formula for sepia tone.
You need to be much more specific. Make what look Sepia? An entire shot? Just a person in a shot? What type of footage? Anything is possible in AE, but based on your question it's clear that you don't have much of an idea how AE works, so perhaps start with the basics:
Quick depends on the shot. Everything about the shot is important. For example if you had a shot that looked like this, you wanted to cut out the guy in the middle with the sunglasses, and it was 10 seconds long here's what you would have to do:
Duplicate the footage, Rotoscope the guy in the middle to separate him from the background and foreground so he was on a separate layer, put that layer above the original footage and apply color controls to the top layer to add a sepia tint. You may be able to use Rotobrush for part of the project but the shallow depth of field would probably require that you do quite a bit of hand roto. The roto for 10 seconds of this kind of a shot could take a half hour or a day depending on the movement in the frame.
A different shot with better edges and less movement would take less time. Unless we have some specifics it's hard to point you in the right direction. If you want to do this in a 10 minute short film it could take months. If you planned the film carefully and photographed each scene with great care and skill it could take weeks. It all depends on the shot.