The first step towards installing mods in Gamecraft is patching the game with the latest release of GCIPA. If you've already done this for another Gamecraft mod, you can skip this step. To install GCIPA, download the latest release and unzip it to where you installed Gamecraft. You can find where Gamecraft is installed through your Steam Library by right-clicking Gamecraft and selecting "Properties...". A properties window should open, with a tab called "Local Files". From that tab, click on "Browse Local Files..." and a file explorer menu should open. When you've extracted the files, you should see three new files, one called IPA (aka IPA.exe), and a new folder called IPA. Drag and drop Gamecraft (aka Gamecraft.exe) onto the IPA file (IPA.exe) to patch the game. To confirm that it's successful, an empty folder called "Plugins" should have been created as a result. If the Plugins folder is there, the game has been successfully patched and you can move on to the next step. (NOTE: you will need to re-patch the game everytime Steam updates Gamecraft)
For this step, you'll need the "Plugins" folder that IPA created for you. Navigate into that "Plugins" folder and unzip the mod there. You should now see a file called something similar to "[name of mod].dll". Depending on the mod, there may be some other file or folders as well, but you don't need to worry about those provided there's one similar to "[name of mod].dll". If you accidentally extract a mod into it's own folder within the Plugins folder, the mod will not work. Your mod is now installed. The next time you launch Gamecraft, the mod will be ready to go. If you open Player.log (found in the %APPDATA%/../LocalLow/FreeJam/Gamecraft/ folder), you should now see the mod's name.