The basic answer is that you choose a subject that interests your child and you spend a chosen amount of time learning everything there is to know about that topic.   You include all subjects into your chosen theme.

The first and most time consuming way is that you can put them together yourself. 

Advantages:

1.       they will be tailored to your family

2.       they can be as cheap or expensive as you choose

Disadvantages:

1.       You have to have an idea of what you are doing

2.       They can drag on you have to have the time to put
           them together




Second you can search the internet and find FREE unit studies that have been compiled by other Homeschoolers.

Advantages:

1.       Mostly free

2.       The hard work is already done

Disadvantages:

1.       You are restricted by what units you can find for free

2.       It can be time consuming, especially if you don’t know
          where to start looking

3.       They might not fit your families ethical / religious
           philosophy





The third option is that you can purchase a unit study
 
Advantages:

1.       They are easier to find on the internet

2.       They are pre-made

3.       There is a wide variety of topics available

Disadvantages:

1.       They can be expensive

2.       You may not be able to find the topic you want

3.        They might not fit your families ethical / religious
           philosophy