After you purchase or download a preset, the creator of the file should give you the .fhp file to add to your game (In the FH folder, where exports, media, etc folders are.) Place the .fhp file with your username into the Presets folder.
If you are given a zip file with the .png and .material files, you will need to go into FeralHeart > My_Presets. Based on the preset number (ex. preset_3, preset_6), you will need to place both the .png files and the .mat files in the right numbered folder. (So if it's preset_3.material / png, you need to put it into the preset_3 folder). It will ask you to replace it, click yes.
Once you have the preset in the right My_Preset folder, you can open the game, and head into the Creative Den, and then Preset Maker. There is a box that says Preset 1, Preset 2, etc. Find the one that has your new preset, when you click it you should see the character wearing the preset. Then click Export, and there ya' go. It will be downloaded into the Feral Heart Folder, in Presets. You may then take your new preset .fhp file and upload it to share with others.
Hope that helps!