I think to get away with it you need it to look like an accident or something to blur the waters and get manslaughter.
The comments about drink driving, dangerous driving etc are also valid.
If you are male you stand more chance of getting away with it, just say the person you murdered was into 'rough sex'.
Mick Philpott tried to murder an ex girlfriend and her mother. The ex, Kim Hill was stabbed repeatedly resulting in a collapsed lung, a punctured bladder, kidney and liver.
Philpott spent 3 years in prison.
I suppose the other thing is to carefully leave a false trail that someone else did it.
So borrow the patsy's phone to research methods, 'borrow' their phone and take it on a little trip. Borrow their car and drive through a couple of red lights disguised as the patsy.
Carbon monoxide would be a good one, a dodgy water heater can produce that. The problem is you need to inhale a well and if you only have a bit in your blood stream then that will indicate you were not in the house.
I suppose if you could get the person exposed and then 'discover' them.
Eg, say it was your partner, arrange a winter weekend away in an isolated little cottage somewhere. Get them there then get hold of a gas heater, start to use it without opening any windows (you may have to bring this with you or have hidden it some where). Turn off the actual cottage heating so the only heat appears to be the gas heater
Then you need an excuse to leave, maybe you have just started your period and need to get supplies. This is why the cottage has to be isolated.
On the way back you run out of petrol so you have to wait for the AA / RAC so this further delays you.
But the plan isn't fool proof.
Your partner needs to not know you have to open a window with those heaters.
Your partner not noticing the heating is off and turning it back on or contacting the owner of the cottage.
Sometimes I worry about me.