Tips for Building Mascots
Yeah, building a mascot is a somewhat daunting prospect, so I'll try to break it down. I'm sure there are other ways to do things, but this is what I'm experienced with, and I know it works.
First of all, draw a sketch like this example, so you can figure out how the mascot will be proportioned to the wearer, and where the vision will be. Use graph paper so you have a standard system for scaling things up. This way you can accurately measure how big to carve the head in proportion with the body.
The head is carved out of hard (ethafoam, styrofoam) or soft (upholstery) foam, and then covered with fur or fabric. Covering the head can be done by hand-sewing or hot-glue; some seams can also be done on the sewing machine. The head is built around a hockey or football helmet, or, if the head isn't big enough to accommodate a helmet, a ring of soft foam inside that keeps the mascot head secure on the wearer's head.
Vision: The eyes or mouth of the character should be at the wearer's eye level, if possible, so that he/she can use them for vision. Window screening, thin fabric or a thin layer of loose-mesh foam are useful for making eye or mouth screens.
Ventilation: Buy a small battery-powered fan with an on-off switch to install inside the head (the head will have to have an exhaust vent for the fan). The exhaust vent can be covered with window-screening (painted the colour of the head) to disguise it. You want the fan to blow OUTWARDS, drawing the hot air outwards and creating a current that draws cooler air in through your breathing/vision screen.
If the character is very bulky, you can build an underlayer of foam padding that is separate from the outer 'skin'. This way you can wash the 'skin' if it gets dirty. Alternately, you can just hot-glue the 'skin', if it's furry, to the foam body. The latter method is less durable, of course, but quicker. The best way is to sandwich the foam padding between layers of fabric which are sewn together at the seams, sort of like quilting.
If the body isn't very bulky, you can just sew a layer of 1/2 inch upholstery foam on the inside of the mascot 'skin'.
Try to make separate components, if possible. For example, detachable hands can make life a lot easier.
Hands and Feet:
Hands can be paws or whatever, but try to make them sort of like oversize gloves so that you can still use your hands a bit if possible. Make them out of soft foam covered in fabric or fur.
If you're making big mascot feet:
Cut out a sole of hard rubber, or scrounge around for something else that would work--car floor mats, perhaps. Glue the sole to a chunk of foam that's at least twice the diameter of your shoes, then carve out the inside so your shoe can fit inside. You may need to glue Velcro straps on the inside to keep your shoe from shifting around, or if you have some old sneakers you can glue them permanently to the feet. Barge (aka shoemaker's glue) is good for this. It's thick, yellow, very smelly and very strong. You can get it at shoe repair places, but only use it in a well-ventilated area.
If your mascot has normal-sized feet: just make shoe or boot covers in matching fur or fabric.
In both cases, the 'foot' can be permanently attached to the leg of the mascot, or can attach with velcro (probably more convenient).
Things to consider:
You will get too hot. Don't wear the suit all day, and take the head and hands off as often as possible. Drink lots of water.
You will need a friend to act as your 'handler'. It can be difficult to hear properly in a big mascot head, and your vision is usually limited to straight ahead, so it is essential to have somebody with you who can field photo requests, questions, etc. as well as help you through crowds and with getting in and out of the suit.
Oh, and you can wash fun-fur in the machine on cold, but don't put it in the dryer or the fur will get all matted and nasty. ^_^ A dog brush is great for making fur fluffy again after you wash it.
Lots more info on making mascots: http://www.fursuit.org/