Hi, I've used protein powder before and after my op.
I use Impact Whey from My Protein. They do small sample packs of all their flavours so that you don't end up with a huge bag of something you hate!
Personally I hated the choc flavours and preferred the fruit or vanilla or plain ones so I now buy a huge bag of unflavoured and a few bags of their flavouring powder in strawbery and vanilla but you could use NAS Crusha if you prefer.
I mix one measure of protein (plastic scoop can be bought from My Protein) with some water (sometimes milk if I want it tastier) and take it in place of a breakfast or as a mid-morning or afternoon snack. It fills me up and stops any snacking.
Others may use it differently but this has worked for me - I don't eat meat or drink milky coffees etc.so would struggle to get enough protein otherwise.
Be aware that there is still calories, fat and carbs in protein powders so use it as part of your diet, not as an 'free' extra.
Good luck.