This is a fun thing to do and can be done either individually or in groups. It can be started without having an idea as to what the final shape will be, and change direction several times before the final giant emerges, but it is a creative effort. It's just required to start.
Two ideas are presented here, both create tall giants, and once the frame is made, then they can be repurposed to be different characters by changing heads and dressing them differently.
It is possible to build and operate a giant by yourself, but it's a lot easier and more fun as a group. It's also a good idea if even if you build it yourself, when you go out with your giant, then someone goes with you to help and keep you and others safe. So a team effort is best.
These giants are using reused materials on the whole, and frequent trips to the tip provides a source of many materials. It feels more fun later if you can tell onlookers that its recycled newspapers, an old duvet and half a tin of paint.
Music with your giant is also good, this can be basic, just a drum, but if you have musicians to play tunes for your giant then all the better. Some giants are even known to dance.