First off it takes 1 recruiting cycle to see the results of your potential redshirt. (ie whenever a cycle would sim, you’ll get your response even if you throw it on 1 minute before said cycle)
Using the Redshirt button during recruiting is the only “For sure way” to make sure a recruit you want to redshirt will in fact accept the redshirt. What I have posted before is just a pattern I have noticed and something you can use to possibly not have to use the button. Again It is not 100% guaranteed.
Another way to get a possible recruit to redshirt is to put them on mop-up and leave them off your Depth Chart for the exhibition schedule. You can also keep taking the redshirt on and off the player may entice him to take it, or it could result in an even bigger Work Ethic drop.
It is possible to redshirt a player you promised minutes to as I have noted in the above link. Although its not guaranteed and really doesn’t make too much sense.