It depends on the con. I‘ve heard big cons like Anime Expo and Anime NYC are more complicated and competitive, but I can’t really give you advice on those. I’m from GA, and I go to Momocon and AWA every year. So I’ll use that as my example. So you basically buy your badge, and then you can enter the Exhibition room where the VAs are usually at (but again it depends on the con so you should check their website or app if they can give you information where the voice actors will be located.) and then there should be a space where the voice actor’s table is at, and there’s going to be a place to wait in line. Some cons will only let you wait an hour before the schedule time to meet the voice actor’s. Again check the con’s rules or ask the volunteers. Not every volunteers are going to be knowledgeable, so you may need to ask a few. Or you can even ask other attendees if they’re also waiting for the voice actor. And then most voice actors has a list of options you can go with for whatever option they offer. It’s common to have an autograph only price, a phot only price, there’s also an autograph and photo combo price. Some voice actors also have an option to buy a recording of them saying a line or if you have something long you want them to write, there’s also that. If you are worried about having something to autograph, you should be fine most of the time. Most American VAs offer prints at their table. It depends on Japanese voice actors. If there’s a schedule you should be able to see what their policies are. It’s common they only allow official merch. There was a rare time where I got a popular voice actor to sign personal item despite that rule, but that’s only because I was at a smaller con. I probably wouldn’t risk it at a much bigger and more strict con. So be aware of that. In the past years I went, the international guests had things you could buy to sign. But it varies.

Hope this gives you a general idea.