Could someone give me advice on how to apply Decals without them looking really shiny. I try my best to make my models look good but decals ruin it for me.

To get the best results, you should glosscoat the model before decaling. Once the decals are dry, a coat of gloss to blend them in, then a couple coats of dullcoat will bring it all back to flat. Weathering will also blend the decals in better.

Thanks, I'll try that.