When you ask how hard it is, this depends completely on the applicant and which program you are applying for.
The most popular programs for applying to Peking and Tsinghua are the Chinese language programs. Tsinghua has a larger program of around 400 students and Peking has around 100. Both of these are filled every year.
You need to make sure you meet the entrance requirements of these programs, apply on time, and send the hard copies to Beijing. Applying as early as possible is essential to increase chances of being accepted.
There are a number of other programs in English and Chinese available at Tsinghua and Peking. The admissions processes depends on which program you are applying for as they are generally managed by each school.
The competition for Chinese students to apply to Tsinghua and Peking is extremely fierce because there are so many students in China and Peking and Tsinghua Universities are the top two Universities in China. Compared to Chinese students the application process for international students is generally easier because there is less competition.
For foreign students the application process is generally more difficult than other Universities in China because the admissions requirements are higher and there is more competition.
I studied at Peking University and my company China Admissions helps international students study at Peking and Tsinghua University each year. We also provide free advice to international students who want to study in China.
You can also watch the video here which talks about PKU and Tsinghua University