I've done this fairly recently.
Dd was demand ebf and a bottle refuser. I started a sippy cup with her at 5/6 months and she'd barely take half an ounce. Started weaning in earnest in November when she was 7m as I had to go back to work in January.
I went from bf to sleep and lots of other bf during the day to being more scheduled, and especially separating feeding from sleeping. I think that's crucial. So I gave a bf first thing, then breakfast, then offered a cup feed, then bf the rest of the day (including to sleep if need be). Then after a week ish I did the same for an afternoon feed, then for the morning feed.
I also stopped feeding at night (slowly extending the time between feeds) as I found she was making up for the lack of milk in the day with more bf at night.
By late December she was just having an evening bf and all the rest was cup feeds of formula.
I won't lie though, she wasn't keen. At that stage she'd barely have an ounce at each feed and I was so worried about how little she was drinking. She started at nursery and was still drinking v little. It picked up slowly though and now at 9m she'll happy down a 7oz cup in just a minute or 2, and sometimes want more.
TBH it wasn't til she got a bit thirsty that she actually decided she'd have the cup if that's all there was.
She still tries to suckle sometimes if she's tired or poorly and I stopped bf entirely at the start of jan. I had to stop for work as wouldn't always be home in time to bf, otherwise I'd have kept up morning and evening feeds. Bit sad really.
Having someone else give the cup didn't help.
What did help was holding in a different position to bf, more upright. Be persistent, don't give in, of course she'd rather have a bf. Accept that if she can physically use a cup she won't starve herself. If the formula is an issue try a different brand - I switched from aptimil to Hipp which she much prefers for some reason.
Hope that helps. It's tough but you'll get there. Personally I'd stay well away from bottles, just another thing to have to wean off in a few months!