gear indexing woes

Is this a shimano thing?

Usually the most common issues, also check housing to see if it broke through.

I'm rubbish at doing my gears.. Rear derailleur issue:

Just stuck a new cassette and chain on my bike - assumed I wouldn't have any gear issues, but now when I try shift to a higher gear (i.e. smaller cog), the chain just jumps straight to the smallest cog - completely skips over all cogs in-between. Moving gears up the bigger cogs is totally smooth and fine.. guessing this may not be an indexing issue..

Have tried googling and watched some indexing videos but can't find this specific problem.

Cassette is bigger than the previous in case this makes a difference. Veloce medium cage derailleur. Think my chain length is okay.

Anyone any idea what the issue is??

At the moment when riding I can get by by freewheeling as I change gear, i.e. shifting down, and shifting up a couple before starting to pedal to ensure chain doesn't jump all the way down.

Sounds more like a shifter issue.

Any shift to a harder gear drops it to the smallest? Whether at top of block, middle etc?

yes exactly, doesn’t matter which gear
I’m in.

I changed my stem so took the shifter off the bars and reinstalled it but didn’t touch the cabling but maybe ive pulled at the cable or something. Maybe I should undo the gear cable and reinstall? Hate touching my gears as I always struggle getting them right again..

Thanks