Puree 1½ cups of cranberry sauce in a blender (you can also use hand held blender) and set it aside.
Add water in a medium sized sauce pan and heat pan over low heat. When water is heated, add agar agar and whisk well for 3-4 minutes. When the liquid starts to simmer, whisk vigorously for 2 more minutes. After 2 minutes, turn off heat, and stir in sugar. Add cranberry sauce to the liquid and stir the mixture until well combined.
Once the cranberry/agar agar mixture is smooth, pour the mixture in a mold and let it cool. Cover the mold with plastic wrap and refrigerate for 1 hour.
To unmold, run a knife around the edge of the hardened sauce and place the mold in hot water for few minutes. Place plate over the mold and turn the plate and mold upside down.