If you'd like to design a Decal, you'll need a Scion Decal Voucher.

Every Scion you purchase comes with a complimentary Decal Voucher.

You can also use the Trading Post to trade for a Voucher.

You might also find vouchers around Whyville, which you can grab and use.

You can check your satchel for any vouchers you already own.

Once you have a voucher, you can redeem it to create a design by clicking on it in your satchel. Studio time costs 100 clams per design. You must produce at least 10 parts and the charge is 10 clams per part. Of course, you can sell your parts for whatever price you want to up to a limit equal to your salary. So, if your salary is 50 clams, you can price your parts anywhere up to 50 clams each. If you do price your part at 50 clams, that means you'll make 40 clams profit on every sale.