There are a wide variety of unit studies and theme units out on the internet. For most topics, you can search for the keyword and tens of thousands will appear for you to surf.
But let's say that your particular child is interested in (as happened to me when my daughter was 6 or 7) Louis Pasteur, but is way younger than the target age group that theme units and unit studies available on this topic are based on. How do you create a unit that fits their capabilities, keeps their interest, isn't over their head, and fits your teaching goals?
To illustrate this, I'm going to use the topic of snow.
First, write down loose categories of what you want them to learn. For this example, I'm going to say that I want my child to learn:
Art and poetry
and to practice their relatively new reading skills.
Next, create subsets within the category. Just brainstorm here and don't worry too much about the "how". Write down everything within the category that might be fun. For science, let's say, I might quickly scribble:
How snow is formed,
Snowflakes - are they alike? how different?
How to make fake snow?
Make a snow crystal?
You get the picture. Do the same for each of the broad categories you wrote down initially.
Now, from the subsets, circle two or maybe three of each, depending on how long you want this unit study to last. If I were going to write a week-long study, I might choose to spend a day on each category, or I might want to choose a couple of different things for each day. In this example, I'm going to decide that because my child is also doing math and grammar, and has a full complement of outside activities such as piano lessons and swimming, that I want to spend two hours per day on the topics within the subject of snow.
So, I am going to divide it up like this:
Each day: One short book, or if my child is a little older, two chapters from the larger book.
Day 1: A snow science topic, snow reading (Snowflake Bentley), and learning a snow poem such as AA Milne's "Tiddley Pom".
Day 2: Snow safety, snow reading, and studying a classic painting such as a Group of Seven artist's rendition of winter in Northern Ontario.
Day 3: Watch a movie based around winter survival, maybe "March of the Penguins". Have the child write or narrate to me what they watched. Make fake snow from a recipe found on the internet, and make a toilet tube penguin to play in the fake snow. Discuss casually, while playing, the tenacity of the penguins despite all obstacles.
Day 4: Snow science - start a snow crystal growing from instructions found in a book. Make a crayon resist painting of a snowflake, maybe using one of William "Snowflake" Bentley's stunning photographs as inspiration, found in his published book. Read.
Day five: Admire snow crystal and hang it in the window. Finish the book. Perform a poetry recital/art display for grandparents while drinking hot chocolate.
Based on my tentative schedule, I will now order my books online, print the recipes and gather ingredients for the science experiments, prepare the poetry and art supplies, and get my camera ready to document it all in a a three-ring folder for my child's portfolio. If we use the notebooking method in our learning, I will also print off some science experiment pages, and some snow-and penguin-based notebooking pages so that they can document their discoveries.
This is obviously a somewhat simplified example, but the basic method of planning can be used for any format and any topic. It will also cost much less than a pre-prepared unit.