How to become a ninja
The quick method: something simple like the sleeve off a t-shirt or a scarf, best coupled with a hoodie and/or sunglasses and/or a peaked cap.
The best method: the demonstrator starts by putting on a t-shirt, but instead of pulling the head hole over their head simply uses the opening as a slit for their eyes, using the shirt sleeves to tie a knot around the back of their head (see picture). This gives complete head coverage except for the eyes which can be covered during daylight with dark sunglasses. If coupled with a hoodie, virtually impossible for a copper to rip off quickly or at all as the hoodie protects the shirt sleeve knot. Add gloves and sunglasses to the mix and they won't even know the colour of the happy protester's skin (racial equality? in the presence of a copper?? whatever next).
General points (aka the small print): Best to use plain black material to avoid your mask becoming a distinguishing feature in itself. Masking up can earn you a very invasive stop&search if a Section 60 is in place (as it's probably best to assume will be the case throughout London for the next few months), in particular when leaving kettles or other flashpoints those wishing not to be searched tend to duck out of sight for a moment to swap outer layers of clothing for those prepared in a backpack and demask, and from then on use hands to hide facial features (s60 only applies to hiding face with clothing). A peaked cap is acceptable to wear in day-to-day situations (unlike a mask) and protects from most CCTV (being higher than your head) especially when coupled with a hoodie. Turn the cameras on the cops - take your best snapper along on a demo and send the pics to fitwatch.