Sam's just started using one in the last couple of weeks. I got him one that he actually had to suck from at first but even I had to suck hard to get anything out and he'd just bite the spout, so I bought him a tommee tippee one - the liquid will just drip slowly if you hold it upside down so they get the idea that they're meant to drink from it, but the spout can be flipped down when you don't want drink thrown everywhere. He's really good with it already - usually he picks it up by the handles and sucks on the lid, but with a bit of help he'll get the spout in his mouth and drink
(I did just hold it for him at first but he'd grab it anyway)
It's the 4+ month one but I think they do a 6+ month one too:
https://www.tommeetippee.co.uk/product/first_cup_4_months_/
https://www.kiddicare.com/content/ebiz/kiddicare/invt/tomtipfstcup4mgreen/tomtipfstcup4mgreen_m.jpg