You can't.

What we refer to as equivalence is only an approximation. You can't put a different lens on a crop sensor camera and get the same shot with the same field of view from the same shooting position with the same depth of field using the same ISO and the same shutter time as you can get with a full frame camera. The converse is also just as true. You can't put an 80mm lens on a FF camera and perfectly replicate the same shot you got with a 50mm lens on an APS-C camera. To get a number of the variables to match, you must be willing to allow other variables to differ.

Having said that, to even begin to get close to the look you can get with a 50mm lens on a full frame camera, you must begin by using a lens with a similar field of view on your APS-C camera. This means a lens with a focal length of around 30mm. And to replicate the depth of field of a 50mm lens at f/1.8 on the FF camera, you must use an aperture of around f/1.1 on the 30mm lens. Unfortunately, there are no 30mm f/1.1 lenses available in the Canon EF mount. If there were any such lenses available they would likely be several orders of magnitude more expensive than any version of an EF 50mm f/1.8. It is quite likely the price difference would be even greater than the price difference between your 600D and a Canon FF model such as the 6D.