there are special tools to do this, but if you don't want to just throw money at the problem and wait for delivery of the tool, it can be solved with simple hand tools and a bit of ingenuity. You could use a pair of needlenose pliers ground to fit around the chain rollers to squeeze the ling together. If you don't want to do that, use a pair of needlenose visegrips or chanellocks to squeeze the link diagonally.