You have to use half-tones to create subtle variations in value. I wouldn't say it is easy if you are an amateur. If you are a screen printer then you know about mesh sizes. To do fine gradations of value and tone you need to be working with a finer mesh. You also need to be able to create half-tones with your computer software. Yeah...this could get boring and too detailed. Simple answer is yes, it is difficult.
I am a silk-screener and we do not reverse artwork. We print film-positives, not negatives. I would ask your silk-screener for art requirements before sending them the file. If you are printing only one shirt, are you sure they are actually silk-screening the T-shirt and not heat-pressing it? I don't know any reasonable silk-screener that will print only one shirt - there is too much set up involved.