If you don't succeed after the first try, put some baby powder inside your cap to have a better grip on it.
Avoid wearing a swim cap with sharp accessories like earrings. Sharp objects can easily damage your cap, making it tear apart. Additionally, be careful putting a cap on if you have long fingernails. Avoid pinching the cap inside.
If one swim cap feels uncomfortable, try wearing two caps with fabric underneath silicone.
If your swimming cap keeps slipping off your head, try putting swimming goggles over it to secure it additionally.
For those with long hair, remember to secure your hair in a bun with a hair tie. Don't rely on a cap keeping your hair inside as eventually it'll fall out of the cap.
If you're struggling with your goggles leaking or slipping, wear them under your cap. That will secure your goggles, preventing the strap from slipping from your head.