Running Instagram contest can be compared to a professional sporting event. You get prepared first, just like a professional athlete, then participate in the contest itself (sporting event), then you analyze your performance (post-competition debriefing, a sports analogy again). The preparation stage is very important, as without proper training no athlete can achieve their goals. There are also logistics issues to deal with (tickets, getting there on time, etc.)
Giveaway contest management works in the same way (not so much hassle, though). You have to decide on the type of prizes, pictures, terms and rules, to come up with a catchy description. Even before you start your contest, you have to decide whether you are going to cooperate with sponsors or bloggers, to make your target customers know about the contest. You also have to write posts on various social media platforms (Facebook, VK, OK, Twitter, etc.) You may even start sending newsletters via email.