You screw the bottom connector (u shaped) into your top and bottom 2x4 of your railing, slid two tops onto the the baluster facing oposite directions, snap the baluster into the u-shaped connectors then screw the tops onto the u-shaped connectors. These are for ¾ inch (19mm) balusters.

I would print these using something UV resistant and high heat (I used PETG). Also rotate the top 180 degrees when printing.

I got the idea from this video but the connectors are like $14.00 and only do 10 balusters.