>>65312893Pomie that's not how you do it.
Ideally you make her talk to you but realistically unless you're cosplaying something niche that won't happen. I guess you could cosplay Sonny and jokingly say "Hey what up my woman" before chatting to her a bit about Nijisanji.
Generally you want to start by complimenting her outfit and asking if it was a pre-made or if she'd worked on it a bit. Even if it was pre-made say something like "Wow really? Well you look great anyway!". Either way compliment her for how it looks but try to lean the compliment a bit into the outfit and don't just say she has nice tits or something.
After that talk a bit about the series or franchise she's cosplaying from ("So hey have you been a Pomudachi long or just recently gotten into her stuff? You know I've been watching her kind of on and off for awhile but initially I think I got recommended her from one of her music videos.."). Make sure that you don't seem like a diehard fan!!! If she thinks you are then she'll think you're just trying to fuck a cosplayer for your oshi! Seem casual!
After talking for a bit say you're gonna go check out some exhibit but you'd love to catch up later maybe over lunch if she's free ("Hey anyway sorry just noticed the time I promised a mate I'd try to get an autograph for him from some artist he likes but... yeah if you're down for it later maybe we could chat more. Wanna grab lunch maybe?").
Then at lunch talk about what you've both done at the con so far and what you're planning to do and ask if she'd wanna check out some of it together.
This is how you date cosplayers. If they're not already taken..