How do I make more skilled workers?

You get more skilled workers by improving your education system. First, bring up the education overview by clicking on the education icon on the bottom bar, this will show you some detail in the bottom right corner of the screen like this:

There are basically two things on here that you're interested in;

• Number of students enrolled
• Education level

If your number of students enrolled is not maxed out, then this is going to cause you some issues. This means that some of your citizens aren't being educated and it will like be down to one of the following reasons;

• Not enough capacity
• Not enough coverage
• Traffic

Drawing on the information in this answer you can bring up the education data layer to see where your weak spots are - where your education system is currently missing, and target those areas specifically to increase the number of students you have enrolled.

For grade school and high school this will mean improving the school bus stop coverage for these areas. For community college and university this means improving your mass transit systems. In both instances, this means improving your road network.

Once you've maxed out the number of students you have enrolled, your education level will start to increase by itself, upto a maximum level based on the educational buildings you have in your city.

Universities provide the highest level of education, and also allows you to research projects to create great works for your region and provide upgrades for your buildings.

A community college provides many of the educational benefits of a university and is smaller in size.

Grade schools and high schools are basically the same building but with different capacities.

Once you've improved the number of students enrolled in your education system and increased your education level, your industries will have the skilled workers that they need to operate correctly.

I was having the same problem in my city, I fixed it by getting more high wealth sims... so I don't think its just the education level, but also the wealth level of the workers. (so build more $$$ parks around residentials)