You have your work cut out for you...Printing 300 for your first opaque ink job is going to be a big undertaking. There is a learning curve to getting a nice even, opaque print, and written instructions will only get you started on the right track. You must practice to get your technique down.
You don't need 2 screens for this job. Just one screen and opaque white ink. Since it's your first time I'd recommend a 110 mesh. Make sure you have a nice thick shiny coat of emulsion and be sure to do your last coating stroke on the squeegee side of the mesh.
Clamp your screen tightly on your press and make sure there is no slop (side-to-side motion when the screen is in the down position.) Use a good sharp squeegee and make a slow even stroke to shear a layer of ink onto the shirt. Flash it just enough so that you can touch the print without getting ink on your finger, and print again. If it needs another flash and print, go ahead and do it.
Give yourself PLENTY of time for this job and get a few shirts to practice on before you go live. The printing will go much slower than it does with dark on light...and there's nothing worse than running up against a deadline and finding you're behind schedule or worse yet, experiencing problems which you'll need to solve before you can resume printing.
