It depends on a few things.
1. Is it standing or sitting gig?
2. Your position at the venue.
If you're front row for either of these you could hold a banner because you can hold it over the barrier if you like because that way the lads see it and you're not obstructing views.
I suggest holding banners up either in between songs (for a couple of seconds) or whenever they ask for banners to be held up (if ever).
Don't have huge monuments with chipboard and fairy lights though.
Use your noggin.
And make them original.
Do me Dougie style, bang me like your drums, Tom's star girl (arrow pointing downwards) etc.
McFly won't be arsed reading them if they're the above lame captions they see at every gig. Use your imagination, keep them short and use big letters that stand out. And good luck.
Tom's read two of my banners. And it gives you a tingly feeling of happiness. He even gave me a thumbs up for the first one. Dougie didn't get the second one. Silly boy.