If you only plan to visit Shenzhen, you can get a 5-day visa-on-arrival at the border. You don't need to apply for a regular visa in Hong Kong (what this write up is about)
You also have the option of visiting china visa free for a few days if you're transiting to another country (like hong kong -> shanghai -> US). More details on eligibility
So only apply to a regular Chinese visa if you plan to travel multiple cities in China for more than a week
How long does it take to get the visa in Hong Kong?
4 business days for regular service
3 business days for express service
2 business days for rush service
Visa fees
Fees vary a lot by country, for example:
US citizens: $1340HKD ($171US) for regular service
Canadian citizens: $850HKD ($143CAD) for regular service
UK citizens: $1180HKD (£117) for regular service
The current fee schedule is here. The first page of this schedule is the default fee for all countries. The second through last pages of this schedule is for countries with specific fees. If your countries is not specifically listed, then refer to the first page
Express and rush services obviously cost a lot more
Payment is by cash or Hong Kong debit card
Steps to apply
Get all the documents ready:
Your passport valid for at least 6 more months and 1 blank page
A photocopy of your passport information page (the page with your photo, name, etc.)
Proof of entry into Hong Kong or any related residency permits for Hong Kong. For most western passport holders this refers to the entry slip they gave you when you arrived in Hong Kong
A photo copy of the proof of entry/residency permit from the previous point
Either a:
Print out of your return flight confirmation and print out of all your hotels or AirBNBs booked (you don't necessarily need to have every single night booked, but the more the better). This is what most tourists use
OR, an invitation letter if you're visiting friends and not staying in hotels
If you had Chinese citizenship before, you need to do one of the following, either:
Have another Chinese visa in your current passport
Bring a photocopy of a previous passport that contained a Chinese visa (photocopy the photo, info, and Chinese visa pages)
Bring your previous Chinese passport and a photocopy of its data/photo page
Book an appointment online for the visa application center, and print out the confirmation. You can book on their English website https://bio.visaforchina.org/HKG2_EN (click "Quick Access", then you'll see the appointment button). It's optional to book an appointment, but I personally did it to avoid the line up that gets very long
At the visa application center, go to the reception desk and get a queue number. The desk will ask to see your passport, documents, and appointment confirmation printout (if you booked an online appointment)
Then, just wait for your queue number to be displayed on the TV screen (sit somewhere you can see the TV screen easily). The TV screen will show you which counter you go to as well. With an appointment I waited for about 5 minutes. Without an appointment you can easily wait 30 min or more
Go to the counter shown, and give them all your documents. If everything is in order, they'll instruct you to pay. Pay the fee, get the pick-up form, and you're all set
On the day specified on your pick up form, go back to the visa center and collect your passport. That's it
Other tips
Try to arrive as early in the day as you can before 10am. Or alternatively after 2pm. For shorter line up