There are a variety of different imposter techniques.
Level of detail is probably the easiest to achieve, but has two main drawbacks. The noticeable 'pop' effect when you swap between low and high resolution models, solved by gradual transitions between LODs. The second problem is that it's MUCH more work for the artist to generate LODs for every single model. There's a GDC talk which about an automated way to generate LODS - "Automated Level of Detail Generation for HALO: REACH"
A simpler technique would be billboarding, where you use only a flat polygon, instead of a low resolution mesh. This polygon faces the camera, and is textured from a render of your model. This method requires having multiple different views of your object, much like how the enemies in doom. You could prerender all your views to a texture atlas. Modern games use this approach, like Far Cry 3 https://www.youtube.com/watch?v=9P3EI17kHaI
You could also have several different static billboards, and combine them like in this unity package
But the best way (which I think you want) is dynamically rendering your billboards, like in this gamasutra article.
This was first done by Jurrassic Park: Trespasser in 1998, so it's not a new technique. Check out the code review here http://fabiensanglard.net/trespasser/index.php Scroll down to the section on wavelets to see their imposters.
Except the standard LOD approach, your first step would probably to find a render texture tutorial. The orientation of the renderTexture camera must be the same as your main camera, but cropped around the gameObject for efficency. You will need to isolate the gameObject(s) you want to render with a culling mask. And set the camera's clear flags so that you get a transparent background. Then render an imposter billboard in place of the original object.