What did Fazullah say about the girls' education? How did he try to enforce this?

In the book, Maulana Fazlullah is a Taliban leader. He gained public support using charisma and public speeches that essentially presented him as a man of the people who wanted to help better peoples lives:

In the beginning Fazlullah was very wise. He introduced himself as an Islamic reformer and an interpreter of the Quran. My mother is very devout, and to start with she liked Fazlullah. He used his station to encourage people to adopt good habits and abandon practices he said were bad.

Of course, this rhetoric hid his true values, which were far more militant and violent. In the book, it talks about how he would often get on the radio and give targeted talks to women, which were meant to control what they could and could not do:

Fazlullah's broadcasts were often aimed at women. He must have known that many of our men were away from home, working in coal mines in the south or on building sites in the Gulf. Sometimes he would say, "Men, go outside now. I am talking to the women." Then he'd say, "Women are meant to fulfill their responsibilities in the home. Only in emergencies can they go outside, but then they must wear the veil."

The influence of Fazlullah permeated through Malala's culture; for instance, when he would come around, the girls were not allowed to go on field trips, as they were not to be seen outside. Eventually, Fazlullah's men started to blow up schools, even utilizing suicide bombers:

Then, at the end of 2008, Fazlullah's deputy Maulana Shah Dauran announced on the radio that all girls' schools would close. From 15 January girls must not go to school, he warned.

In short, the answer to your question is that Fazlullah said that girls should not be educated and should remain at home at all times. This was enforced through brutal militant actions, such as bombings, as well as pressure put on communities by violence and implied violence.