STAMP filter not working properly

Are you using exactly the same image as in the tutorial?  The background is obviously going top make all the difference to the end result.  You could use Remove Background and Apply Layer Mask to get rid of the background, and then Stamp would only be applied to the subject.

I started with a plain almost white background

So I still had a clear background after applying the Stamp filter

It's nearly always a good idea to make the layer a Smart Object when applying effects, because you can fine tune the result.  Double clicking  Stamp would open up the settings for adjustment

But you could also edit the tonal values of the image so that Stamp can closer to what you want.

I double clicked the smart object to open it in a new window and used a curves layer to get rid of a lot of the darker tones

Which makes Stamp give you a completely different result.  Remember this is now completely non-destructive so you can go back in and fine tune the settings.