This is going to be a step-by-step tutorial concerning the technique of adding new material to the repositories. It explains what needs to be done from the beginning to the end to make a new submission appear on the frontpage in the right category. All you need is a graphic program like GIMP or Photoshop and little knowledge when it comes to HTML and webpages - not too difficult, so don't worry or get scared away, it's as easy as typing a new thread in the forum.
Well, below you can find the guidance for adding an article, but who decides which submission is worth being added? This is exactly the freedom you have. Check the code, check the data, check the art - and then it's time to make a decision. Often it is not necessary to make that on your own as you can get in contact with other team members or even the authors themselves to suggest improvements or ask for a second or third opinion. The only thing that's important to look for, is if the submission is complete (gldefs, sounds, credits) and working (no bugs, no errors, no console-hints). From then on, it's up to you what's worth being added.
Each of the special icons (the latter 5 ones from the complete list of topic icons) have a specific meaning. Users are encouraged to use the first two of these. If they don't choose the right at the beginning, I recommend moderating these (if someone posts an "Approved" topic and it's actually a submission). After your check, you can change them to pending (if the user has to fix something), approved (if it is okay) or rejected (if it - well - sucks). So what are the meanings:
Important Info: Make sure the file has an unique filename, otherwise you will overwrite old files. My hint for this: Add the current date to the file, e.g. "monster_24052016.zip" (24th of May 2016) to make sure the filename is unique. This is only for new additions, not for updates of existing entries. These MUST be overwritten to avoid redundancies.
Editing the text is pure easy, simply get the info and credits from the forum submission thread and paste them into the editor. You can either type it by hand or copy and paste it from the forum (as long as the formatting has been done the right way by the thread author, make also sure to copy clean text only). Otherwise you have to fix that manually as well. Text is easy. Start with the text.