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If you are squeezing out the O-rings, you have not created enough of a pre-flare on the first end to hold the O-ring captive.

It is also helpful to make sure the pre-flared end does not rotate as you are tightening the flaring tool.

Be sure to lubricate all parts and washers on the flaring tool before you assemble.

Or you can just use the air hammer technique, which is even easier.