Obviously, making a full costume from scratch requires a lot of dress-making (sewing) skills. However, many theatrical costumes are made by adjusting existing garments to suit a particular character, and by adding various accessories and hats.
You can copy and paste the body outline images provided below onto a Google or Word doc and then print out.
Add on your chosen garments, colour in your designs and label them.
You can also add swatches of fabrics to show what they would be made out of.
Some videos and images of how costume makers go about designing and making costumes are given below
Costume Design for NT production of 'Jane Eyre'
Making the costume for the 'Light Princess'
Costume Designs for 'Wicked'
The website page below has some very useful information about designing costumes.