install wechat on your phone
create an account by using your phone number
within the wechat app create a userID and password.
log on to wechat on your tablet using your username and password


For the last step you have to look at the login screen, there is a word or a button or something you can click to get the option to log on without using a phone number. You will still require a phone number to create your userID, but its not required to use wechat afterwards

Once in China and you get yourself a chinese sim card you may want to change the phone number associated with your account to your new chinese number. Then you can use it on your phone while here. You don't exist in China if you don't have a phone number - its almost as important as a passport, maybe more important