You'll need to cut the chain to the correct length. Having a chain-breaker tool is nice.
In a pinch, you can use a grinder to grind the head off one of the roller pins and then tap it out with a drift punch. But, if you're not experienced with grinders and hand tools, this can be dangerous if you're not careful.
But that's how I've done it when a chain-breaker wasn't available.
Don't forget to get a new master link also...