"Should I expect them to leave the donation first or at the end?"
Do you want your money to be safe or do you want to be safe yourself?
If you don't get the money up front, he's more likely to skip out without paying.
However, it's a lot easier to end the session and get rid of him if he hasn't paid yet. If you have his money and he's a scumbag, he may fight you to get it back, or feel more entitled to get what he wants.
Cops love to be able to say that you asked for the money, handled the money, or at least that money was exchanged.
It does look more high class to not ask for the money first.
If your business is repeat clients, not just one timers, it shouldn't matter.
"How do I gently remind guys that their time is almost over without sounding like a clock watcher. "
Tell them. Your call as to how close to the line you want to go before mentioning it. If you want to wait until the very end to mention it, you risk a little extra time "to finish up," but it might be worth it.
It helps to have an easily readable clock that can be seen from where you are.
"So when a client first comes to my door, I answer it and then I get all nervous because I don't know how to transition from "hey there, how are you," to the down and dirty deed of the encounter.
Do you all have any suggestions on how to gracefully get down to business?"
Be ready to get down to business as soon as the clock starts, but take guidance from the guy. I don't always want to start right away. When you're ready, give him "the look." Maybe a gentle hug and a kiss, then back off a bit and let him take the lead. See if he seems to be ready to start right off or if he wants to flirt a little.
I love to start on the couch. Sit next to him, and start with some touching or kissing, letting him give you clues as to how far to go. Maybe guide him to removing your clothes or touching you.
Once you get to a certain point, you should be over the clumsiness and be ready to talk about activities.
"What outfits are most desired?"
I like sexy, Some guys want naked. Even those of us who like sexy aren't that interested in "fancy" or "high dollar." Once again, try to let the guy give you clues what he wants.
Me, I like sexy lingerie, silky thigh highs and a garter belt. (Of course, you can wear that, too if you want.

JK, guy stuff only for me.)
I prefer fully dressed to start. Nothing fancy in the outside is necessary. A robe or some sort of "over lingerie" is good, too. Unwrapping is part of the fun. Don't be too quick to undress. I prefer to undress you myself. Also, don't remove everything. I may prefer the bra to stay on the whole time. I definitely love for you to leave the garter belt and stockings on the whole time.
"Also, what do I need to provide as far as toiletries and refreshments are concerned to make a date more pleasing to the client?"
Bottled water is nice because it removes some concern about being drugged. Nothing fancy, but is good to have handy after activities start. Soda might be nice, but there's more of a spill hazard. A resealable bottle would be good. Disposable cups in the bathroom would also be useful. Be sure the bathroom is adequately stocked with clean towels, washcloths, towels, etc. Mouthwash and cups would be nice. Baby wipes at the activity area might be nice, but be sure they don't leave a bad taste behind.

Paper towels are good, too, as well as a nearby trash can.
Truthfully, as long as the place, especially the bathroom, isn't a disaster, we don't care much, but it adds to the class, and the easier it is for us to get clean, the more likely we are to get clean. Personally, I like a quick rinse for the boys before I start.
"Music or not? What genre?"
Nothing too wild, nothing too loud. Somewhat gentle 60's or 70's for me. A lot of ladies use a playlist on their computer as a sort of clock. They know song X starts at 25 minutes. Or you might let the music run out at 25 or 50 minutes to give the guy a clue that time is about up.
No gansta music.
"Can anyone explain screening? Why should I do it? How do I do it?"
Whatever you do, don't put too much faith in it. A guy might be nice to 9 girls out of 10 and kill and eat the 10th one. Realize that the tougher the screening, the less business you will get.
"Is it appropriate to ask a guy his first name?"
Are you talking about at the actual session? Say "Hi, I'm Jane." Use your stage name. If he doesn't give his name, ask if you want to. Expect his stage name.
If you're asking about screening, that's another question.
"What questions should I ask when a guy calls for a date? I don't want to put them off with a barrage of questions."
Handle the mechanical details of where, when, how long, etc. Play the rest by ear. Asking explicit questions may make him suspicious. Maybe ask him about any special requests.