Tsundere must face and realize her own feelings. Realizing how you yourself feel about someone else is very important. So is admitting those feelings to them directly with your own courage and conviction. No middlemen to muddy the message. Hell, it may not even get through to the other party that you're serious, as I've seen in some other works. One male friend support character of the main couple for another work tried to tell the male lead that the female lead liked him back, but he just refused to listen, and insisted it had to come from the female lead.
One of the best stories I've read of late had a similar thing going on, too, where the male lead had to realize his own feelings in his own way, though the fact he had feelings for two girls complicated things so he needed some prompting from his oldest friend to realize what he was feeling for the second girl was, in fact, love, because it wasn't the same as the crush he already knew he had for one girl, and said 'second girl' he spent so much time with, and the feeling was such a gradual growing 'background radiation', if you will, he hadn't had the time or inclination to realize what he was feeling for her, let alone the fact it was more intimate or strong than the 'crush'.