Write a function that takes an int argument and doubles it. The function does not return a value.

Here is an example of a function in Python that takes an integer argument and doubles it without returning a value:

```python
def double(num):
num *= 2

# Usage
num = 5
double(num)
print(num) # Output: 10
```

In this code, we define a function called `double` that takes a parameter named `num`. Inside the function, we multiply `num` by 2 using the `num *= 2` statement. However, we don't explicitly return a value from the function. When we call the `double` function with a value of `5` and assign it to the `num` variable, the original `num` variable outside the function remains unchanged. That's why we see `10` printed when we print the value of `num` after calling the `double` function.

To write a function in a programming language that takes an int argument and doubles it without returning a value, you can follow these steps:

1. First, decide on a programming language you want to use to implement the function. The syntax and structure may vary depending on the language.

2. Define the function with a proper name and the argument of type int. For example, in Python, the function could be defined as:

```python
def double_number(num):
```

3. Within the function body, multiply the argument by 2 to double it. You can use the * operator for multiplication. For example, in Python:

```python
def double_number(num):
result = num * 2
```

4. Since the function does not need to return a value, you don't need to use the return statement. However, you need to decide what you want to do with the doubled value. For example, you can print it to the console. Continuing from step 3 in Python:

```python
def double_number(num):
result = num * 2
print(result)
```

5. You can now call this function with an integer argument, and it will double the value without returning a value. For example, in Python:

```python
double_number(5) # will output 10
```

Make sure to replace "language" with the actual programming language you are using to implement the function.