It is not usually recommended to attempt this
Apart from cosplay gatherings, unless you are willing to approach potential friends and manage to obtain contact info (like Instagram name) in a single meeting it is not likely to happen. Most people come to conventions with friends or to meet friends they already have. It may feel quite difficult to approach someone who appears to be with a large group of friends already. However, you might try posting on the Facebook page so you can make friends prior to the convention and have people to meet there.
If you do go to cosplay gatherings, that is the easiest method. At that point you know you already have something in common with the person and thus an easy conversation. As long as you are polite, there should be no issues, just remember the potential friend may be just as or even more nervous than you. You do not have to be cosplaying to attend the gatherings! Just make sure you actually do follow up with people once you get the contact info (or remember to actually tag them in photos) or have a plan to meet again at a certain panel or event. It is really no different from making friends in any other situation, you have to put in effort either way. If you are shy, go ahead and practice a few conversation starters and make sure to smile when appropriate and make occasional eye contact. Keep in mind your body language and look approachable.
Make sure your hygiene is on point too. I have totally not texted people back over bad breath-- you want to make a good first (only) impression. You could also try various methods to draw people to you like playing games or something.
Happy hunting!
