I'm not being a wiseass, while the "rag-joint" gets easier to deal with the more you build them - you'll never get it perfectly centered and there isn't really an "easy" way...
Having said that, my method is simple - I mount the assembly as "centered" as I can and tighten the bolts snug but not tight, spin the wheel and hold a marker near the sprocket. Keeping my hand steady (that's the hard part lol, brace yer hand on the frame if needful) I slowly bring the marker in till it just touches the high spot. I then tap the high spot using a block of wood to protect the teeth. Wetting the rubber bits with water to help them move can be helpful. Repeat as necessary...
Be warned though - not all these sprockets are made correctly, my first one was stamped offset so there was NO getting it "centered" meh...
(sprocket should be dished out or in - depends on motor alignment & chain clearance)