The often-cited conversion between gelatin and agar is eight to one. meaning agar is 8x stronger than gelatin. However, they’re both natural products and, like all natural products, vary in strength from one manufacturer to another. The only way to know how much agar to use is to evaluate a series of experiments with different amounts of agar.
You would mix it in the whipped cream just as you do with gelatin. But first, what form did you source? If as strands or sheets, these need to be soaked just as gelatin leaves must, then wrung out before use. If powder, follow Rose’s recipe for whipped cream stabilized with powdered gelatin (p444 RHC) for your experiments to test the strength and see how much to use. You could use half the recipe for full strength (4 tsp or 8x 1/2 tsp gelatin), the other portion for half-strength (2 tsp), and see how they each turn out.
btw, agar in all its forms gels quickly as it cools, much more quickly than gelatin. It does give moderate to high clarity, but can give quite a bouncy gel if you use too much. Not what you want in whipping cream. It’s stable at room temp and in the mouth. Works well with acidic fruit like pineapple.