First off, cut the Butternut Squash in half and remove the seeds and pith. Remove the tough skin with a peeler until you're left with nothing but lovely orange flesh.
Dice this up into 1 Inch/2.5cm cubes (or as close as you can get) - it doesn't matter too much if they're too small, just avoid monster sized chunks as these won't soften as well when boiled. Follow suit with the Sweet Potato - peel and dice into similar dimensions.
Place both the squash and sweet potato into a large saucepan and cover with water.
Lightly salt the water, bring to the boil and then simmer for 15 minutes or so until the cubes are nice and soft (the usual test of seeing if a table knife will slide easily into a cube did the trick for me).
Once the cubes are ready, drain off the water.
Add a splash of milk (not too much - you can always add more later) and begin to mash with a standard potato masher.
Once the mashing is getting well under way, add the rest of the ingredients. I tend to split the butter up to ensure a good spread through out the mash. The spices are to taste really - I start off with roughly teaspoon's worth of nutmeg and cinnamon and then incrementally add the chilli powder until it hits that ideal hot spot . Experiment and have fun!
Simply season with salt and pepper, again to taste.