Pretty sure last time mirtazapine may have been an issue, it takes a while to leave your body.
But, you can get nauseous from the nasty syrup , only way to avoid is taking capsules of pure DXM or capsules of ground up polystyrex beads... but in my experience once those open in your stomach it is still very nasty. You can also get nauseous from dizziness and possibly serotonergic effects.
Weed is the main thing that eases these rough edges, but unless you're a daily smoker or something anyway it is not really recommended per se for an inexperienced tripper because it intensifies effects and makes them more unpredictable. Synergizes well though.
If I were you i would eat a light meal, but not directly before - things like crackers with little fat in them etc, just look up things that are digested lightly and are composed of simple ingredients, nothing fermented etc. The general idea seems good to have little going on in your stomach but completely empty may make the DXM land a bit hard.
The dosage is completely up to you - just check out recommendations for the plateaus and leave out complicated regimens like the 150 mg followed by portions later etc. Just take it in one go (but slowly), or in two parts separated by not too much time. Otherwise it may screw up the timing and make it really hard to follow where you're at. If higher doses make you anxious to take just like that, just aim a little lower.
Mindseye: people respond very differently to DXM, it has a lot of side-effects for many people but if you tolerate it really well it has potential. If not, like for me, it's one of the worser ones.
Not on the diphenidine / 3-MeO-PCP side of the scale in my opinion.. it's more holey in ways, like ketamine, and quite physical. Debilitating, feverish with an euphoric edge like tramadol but less euphoric IME. Dirty and confusing which makes it the opposite of subtle like some of the more functional dissociatives can be, or seem. I don't think that things like diphenidine or MXE etc have even close to this tendency of e.g. making you dizzy and everything blurry and soupy. Sure they fuck you up, but in a more clean way.
If you're not on any medication I guess for a disso-head it couldn't hurt to try, but be prepared to be one of the people who don't tolerate it too well, could be sickening and unpleasant.