I am not an expert at Do it Yourself decorating boots, however I did try my hand at my Monday boots last year (under advice from Afro Chic) and here are some pointers to assist you when trying to do the job on your own.
Spray Painting:Make sure and choose the right spray paint for the job.You can also use fabric paint or fabric spray paint to change the colour of your boots if it is faux suede.
If going from a light colour based boot to another colour or metallic use two coats of spray paint.
If going from a dark colour based boot to another colour, spray the boot with a primer first in a lighter colour, for example white spray paint, and then spray it the colour you desire. Over all this may require four cans of spray paint; two as the primer and two to get the desired colour.
Use masking tape to protect areas of the boot that you do not wish to change colour, for instnace the sole of the boot or hardware on the boot.

Spray the boots during the day in natural light in an open area and preferably when it is dry and hot outside.
When spraying try to use even strokes and cover the boots completely, layering the spraypaint to get the right colour.
Decorating Tips:Wait until the boots have dried completely before attaching any decorations if they were spray painted.
Plan your design before applying decorations ad hoc; look at the costume and mimic designs that are on the bra or belt or headpiece if you need inspiration.
You can attach decorations using a hot glue gun and glue sticks or an adhesive specially used on craft items.
Choose gems that are flat backed for easier application. You can also attach feathers behind the gems and then apply the gems to the boot to create more interest.
Remember you can re-use the foot pieces that will come with your costume as part of the boot decoration. I prefer attaching the foot pieces by advehsive instead of wearing it on the boot as is, since it tends to detach and "float" on the boot instead of looking as if it is part of the design.
To apply glitter to the boots use an adhesive spray, then layer on the glitter to your liking.

Finish off edges and seams on the boot with a strip of sequin trim or braid or even beads.
Change the laces on your boot to match the colour it was changed to if applicable.
As a side note you can also apply these tips to decorating your sneakers which can also be spray painted and decorated to match your costume.