How do I model from a side view with a reference image? I know how to setup it up, but if I for example, want to model a car, I will use planes and model it with a  reference image on the back. But then, from time to time, I switch off that view, and I realise what everything I was doing, is looking at the wrong way, and the planes extrude inwards except upwards... What am i missing? should I hold something when extruding? Or is there anything I should know? Maybe it's too advenced for someone who started recently but I would really like to learn how to design like that.

Feel free to share any tips
You need to use background images either in orthographic view, or using stationary camera views rather than planes. Background images is in the properties window on the right hand side when you're in 3D view (you can toggle its view with the "n" key). from there you can load up images, and choose which views they are visible from. all views except for camera view are only visible in orthographic mode which is the view when you press 5 on the num pad and then press 1, 3, or 7 to see front, side, or top views. since things are not orthographic in real life you can also use a camera view background image to keep the reference image stationary against the  model.