Drag & Drop (you can drop files directly onto the game window to load mods, hairs, backgrounds etc.).If not then I suggest starting out with this guide (and getting SDTL 2 once you get the hang of it). If you are familiar with SDT and ModGuy's Loader then this is for you.
SDTL 2 is targeted towards experienced users (for now). It does not replace the vanilla game or ModGuy's Loader! SDTL 2 is a replacement of the Flash projector. It's built on top of Electron (a custom version of Chromium) which opens up a ton of new possibilities. It's a desktop application that you run the game with instead of the Flash projector or a browser. SDTL 2 is an open source launcher for SDT.