Heres a good video showing how its made, however this is not my recipe - the one in the video tastes like shit but the process to make it is the same.
Protein Fluff V 2.0! Monster Volume! - YouTube
Its definitely not sludge, sludge isnt very filling in my opinion.
I use:
300g mashed berries (usually strawberries for the lowest calories)
2 scoops protein (66g)
1 cup of water
1/4 tsp guar gum (too much of the gum like he used in the video will fuck your stomach up)
Extra sweetener/flavorings.
Usually I taste it once its fluffed up and add everything as I go. You can mix in some chopped nuts for fats or puffed cereals to add texture without adding too many calories. The base recipe (strawberries + protein) is 325 calories with my protein powder (110 cals per scoop), and it will fill your stomach like crazy