Add the porridge oats, skimmed milk powder and brown sugar to a blender and pulse a few times so that the porridge oats are fine and everything is well combined. You still want a bit of texture, but you want it fine enough that the hot water will penetrate each grain easily.
Add your choice of optional extras, stir well, place in individual plastic bags and seal. I used a vacuum packing machine to suck the air out of my sachets to maintain freshness, but this isn't required if you're not travelling too far.
To prepare the porridge, empty the sachet into a bowl. Boil your kettle and cover the porridge in boiling water - approximately 120 ml. Cover your camping bowl with a camping plate, and leave to sit for approximately five minutes.