Like any clown, you'll want to start off with a painted-white face. The base used on Skaarsgard in the film was Kryolan Supracolor Clown White ($7). Sansom says you can apply it with a disposable sponge and it will last all night because it's sweat resistant, but the company recommends you set it with a powder because it's highly transferable.
To give the base the weathered, cracked look that Pennywise has in the movie, you can use an old toothbrush to splatter on grey paint.