First thing you would want to do is find the kind of costume you want to create. You can look through images and pictures untill you find one that you like the most. Make sure you take your measurements so you can get the right amount of material for your costume. You would then need to get patterns for your costume. You may need more than one and if you cant find a certain pattern, find one close enough that you can make adjustments with. Next you need to buy your fabric and thread for the costume. I would suggest getting a bit more than what the pattern calls for so you have room for mistakes. As a precaution, I would also wash and dry the fabric before using it in case there is some shrinkage. Iron the fabric so it is smooth enough to lay your patterns on and cut out. Start sewing your pieces together and once you have finished, try the costume on for a fitting. this is where you will see what adjustments you may need to make.