depends on the contest.
grower of the month, you simply answer questions and you will show up in the leaderboard when you get a solution from a question with a journal attached. if no diary link in question, no journal attached.
others you click the "join now" button, accept terms, then select an eligible diary.
rules vary for these a bit, but in general it needs to have a completed harvest week. Sometimes there are date restrictions. Can only submit a diary for certain contests, like "diary of the month" one time, then that diary cannot be used again in those types of contests... 1-time submission only in those cases, but it might be useable elsewhere still.
sometimes these rules are 100 % not enforced. For example, the date requireent is often never adhered to. Whatever variables are used to qualify this stuff seem to get reset occasionally, maybe due to site maintenance... but, i've had prior used diaries become eligible again a year and a half later.
the best is to let a good diary simmer for a while... collect a shit-ton of views and comments etc... then start using them in contests. I don't do that, but it's a wise strategy if it is important to you to maxiize submissions. And, like i said above, you'll often see diaries that have already won something re-submitted.. eventualyl, unless they fix it, it'll becoe eligible again for submission in a year or two.
when you win, a little dialog window will pop up asking for "tshirt size' and verify name/address etc. Good idea to type in any customization that the prizes allow in the comment box for tshirt size. This process does not even start until the contest finalizes and is official.. so don't hold your breath. If it's the ides of the 'next' month, maybe a little email is warranted. figure 2 weeks to finalize stuff or more, then a month after that... if i don't get a notification, i start snoopin around.