door knob

I have a Upvc front door with lever handles inside and out. Unfortunately our husky now knows how to open doors and has started letting himself out. I don't like locking the door because kids, postman etc. tend to come and go during the day so I'd like to replace the inside handle with a knob...

Hello, I've brought a door knob to fit to an old rim lock. It came without any instructions and I can't work out what the circled components do (see picture). It seems to fit using just the door knobs and spindle. I cant work out what the collar and 2x plastic washers do. Any ideas? Cheers.