Chinese characters are fine.
喂。我想为上午10时30分星期六点心保留。
i would say it this way:
麻烦你一下, 我想定位子吃点心. 星期六早上10点半.
I think the above replies address the OP, but in my experience, all you need to tell the restaurant is when and how many people, period. Obviously they serve dim sum, so you don’t even have to mention that in your reservation. Just say I want a reservation for x people at <state the time and day>.
My mother really loves, LOVES, spicy food. Whenever we eat Chinese food (which is fairly often as we are stereotypically Jewish) she orders a spicy dish and asks them to make it extra hot. It seems that most cooks think the average American cannot handle truly spicy food. She always has to send it back once or twice and ask for hotter.
While you’re all here, could I get a sentence meaning roughly ‘Please make it as hot as possible. Do not worry that it will be too spicy for me. Thank you very much.’
我真的要吃辣的菜. 不要老外了辣, 要四川辣.
I would say it as “I really want spicy. Not foreigner spicy but Sichuan spicy.”
If they take that literally, it will be so spicy as to burn on both ends. (only caveat is that Sichuan food depends not only on nuclear fire but also Sichuan peppercorns for the numbing effect. I personally can’t stand the peppercorn taste but many people love it.)
Out of curiosity, what is the context in which your server won’t understand the same sentence in English, you don’t speak enough Chinese to know how to say this yourself, but the phrase written out in characters will be useful to you? If you’re gonna print it out and give it to someone, wouldn’t you also need to tell them which dish you’re referring to?
The server can understand it just fine. But, experience has shown that they don’t believe it. It is my hope that by showing she cares enough about getting truly spicy food to print out a card, they will believe her and deliver truly spicy food.
Thanks, China Guy I e-mailed the post to my mom.
opps, I had a typo. Should be:
我真的要吃辣的菜. 不要老外的辣, 要四川辣.
Richard Parker - it’s like telling a barber I want it really short. I mean butch clippers, marine boot camp 1 mm long kinda short. They invariably will maybe trim it down to about half an inch, then a quarter inch, then they sorta kinda believe you and take 'er down.
If it’s just the normal level of spiciness like you get in Sichuan, Hunan or Guizhou Provinces, most gringos will find that inedibly hot. Seriously, 2 bites, the sweat is pouring, your mouth is numb, and 5 cold beers later you’re still not ready for the 3rd bite hot. And if you do get through the meal, then a lava bullet train has left the station and will be burning through in short order. Most restaurants don’t want that to happen. 
Why does handing them a printed sheet that she does not understand make her more credible than if she just explained in English? I guess I just don’t share the intuition behind this plan. But I’m curious to hear if it works!
I would teach her to pronounce: 如辣不够, 那钱不够! [My Chinese is a little rusty, but something like “if it’s not spicy enough, then I don’t have enough money (to pay)”]
