Chain slips on low and high gear and sometimes when putting some strain into it I have been running an 8 speed cassette/freewheel, don't which cuz I never checked before it seized the other day.I was using it with a 7 speed shifter, not ideal I know but it worked OK just used 7 of the 8 cogs.

As mentioned above, the other day my cassette/freewheel seized. I had a spare back wheel with a 9 speed cassette on it (I learnt how to check yesterday) so I whacked that on and thought I could just use 7 of the 9 cogs. The chain now slips at both high and low end and sometimes when putting extra strain on it.

So my question is, what did I do wrong and what do I need to do to correct it please? Btw the rear derailleur is an 8 speed and the crank set is 3 rings, if that helps any.

Thanks in advance XD
Both wear of the rest of the drivetrain (chain in particular can be stretched) and chain width mismatch (9 speed chains are narrower, and so are distance between cogs in the freewheel) can contribute to the skipping you see.

In general, 9 speed freewheels work neither with 8 nor with 10 speed chains.