Hi there I recently bought strings to replace the ones on my yamaha. I've changed strings before but I have no experience with pins.The pin keeps popping up when I start winding the string. Is there an easy way to do it? How do I know when it's securely in there?
Are you sure you've inserted the string into the bridge correctly?

Insert the ball and a bit of the string into the bridge. The pin should have a groove in it; place the pin in the bridge with the string in the groove. Now one you've placed the pin in, don't press it all the way down - pull up on the string so the ball catches on the pin, against the inside of the bridge. Now that the string ball has been set, *firmly* press in the pin. Now begin stringing the tuner. When winding the tuner, the tension of the string may start to pull the pin out slightly. This is normal - just push it in firmly again until the pin is stationary. Continue to string the guitar.
Yeah what phat said pretty much, you want to be careful about your pins on acoustic guitars though.

Making sure the ball of the string sit right at the bottom of the pin before you push it into your acoustic is important.

When pushing the pin in, you may want to get part of your hand in the opening of the guitar to give you a little play, so you can push harder without damaging the guitar.

its just kinda somthing you gotta **** with, but dont push to hard without any support or you could damage your guitar.

I use a quarter.