Read the above post. Then read it again. You will be grateful for not having to deal with Guaifenesin-induced nausea, as DXM alone can be pretty stomach-bending for some folks. Make sure to acquire syrup with DXM as the ONLY active ingredient; if you can't get it, then don't do it, its not worth the risk. And, if you have the money, invest in an anti-nausea aid like Dramamine 2, because the nausea can be be brutal and take you by surprise.
As far as dosage goes: Measure your dose in mg, and probably don't aim above a medium second plateau for your first experience. Since you haven't stated your body weight, there is no way of knowing what would be a good dose for you. If you intend to take DXM, here is some required reading:
I've used guan containing dxm and went up to 7+ grams didn't get any sicker than with other OTC dxm products and several other robo-junkies I've talked to say the same thing.
Ive never experienced any ill effects from guaif but in most cases people report intense nausea. I would pass on that if I were you and find some robogels or any other dxm-only product.