What the others said. Also have the lower in a vice (using the block). I will also put a small pin or punch through the bolt catch holes to hold it in place while i drive the roll pin in. As the roll pin goes in it will drive that other one out
I put some of the thick clear 3M packaging tape on either side of the boss to protect the lower. I put a punch in the opposite side of the roll pin to hold the bolt catch aligned and in place. Then I use the half flat roll pin punch and a small 4oz ball peen hammer to drive the roll pin in. The half flat punch is made specifically to install the roll pin on an AR. Or you can just spring for a lower with the screw in "roll pin".
As with anything you do in terms of building an AR using roll punches and a hammer...if you have to use anything more than some very light taps with the hammer you are doing something wrong. There is nothing on any AR that requires a lot of hammering....tap tap tap tap tap...should get it done regardless..
I have found first and foremost that an lower receiver action block clamped in a vice is essential to doing things like this pin. Holding your work steady is crucial.
Second...you should only need to use a little blue painters tape (or any kind really) on the receiver to the right of the pin hole assuming you are right handed. I hold the pin with a needle nose pliers making sure you start the tapered end of the pin first and use a little oil and get it started just using a small hammer and the needle nose. Once you get it started you need to use a roll pin punch to drive it home and it might take a small amount of fidgeting and positioning of the bolt catch assembly before the pin aligns--i usually drive the pin in up against it with the slightest of tension holding the assembly in and just wiggle it until the pin drops into the hole. Then tap it home. A little finesse... As I said. light taps.... tap tap tap tap... Its not hard after you've done a dozen or so...