A man talk to a cake seller: please make me a cake ,just a cake. then that man has 订做了 a cake. another man talke to cake seller:please make me cake,it must be rounded, 100mm high.smelled like chocolate.then this man has 订做 and 定做 了 a cake. and that's many chinese 's proplem.people always confuse them. for example:我在网上订了一张车票，已经定下来了。but people always say 我在网上定了一张车票。imply you can get that ticket for sure. ps. 度身定做 not 独身订做。 the primary meaning is making clother according to your size.so use 定。

“定做”: you have a fixed time to let some people make something.and this thing must be finished. eg: you have my new clothes made to order . "订做": you bind some books,newspaper ..or make a plan or treaty after the discussion. maybe I might be help you in this answer.