1 - Open Adobe Illustrator.

2 - Create a New Document or open an existing one.

3 - (Optional) Go to View > Rulers > Show Rulers or click Command/Control + R to turn on page rulers.

3a - (Optional) Click and drag on the rulers to place a Guide on your desired position (In this case we placed it 1 centimeter from the top). Repeat with the vertical ruler. (Go to View > Guides and make sure that "Lock Guides" is selected so you won't select and/or move your guides by mistake).

3b -(Optional) If you want to modify or delete your guides you will need to "Unlock Guides" or "Clear Guides" in order to do it.
4 - Choose the "Rectangle Tool" from the Toolbox and draw a square by clicking the "Shift" key while pulling the cursor down and right to make it's sides even.

5 - You can change the default "Fill" and "Stroke" colors in the Toolbox.

6 - Select your square and go to Object > Move, or click Shift + Command/Control + M. In the "Horizontal" position box add a value equal to your shape's width (You can check it in the right side of your upper menu. Add a value of "0" to the "Vertical" and "Angle" boxes and click "Copy" to place a copy of your shape in the desired position. (If you want you can check the "Preview" box to make sure your object is going to be placed where you want). *


7 - Go to Object > Transform Again or click Command/Control + D to repeat until you fill the line of shapes.


8 - Select your objects, go to Object > Transform and this time change the "Horizontal" value to "0" and the "Vertical" value to the height of your object. (The "H" value in your top menu). Repeat using Command/Control + D as many times as needed to fill the page. *


9 - Select some of your objects (In this case we selected alternated squares) and go to Object > Transform Each. In the appearing dialog box choose the values you want to modify. We chose a "Scale" of 80% both "Horizontal" and "Vertical", a "Rotation Angle" of 5 degrees and checked the "Reflect Y" box. Click "OK".



10 - End result.

This can be particularly useful if you want to create vector textures.
* Note: You can use a bigger value if you want to have some white space between your objects.
by @lornacane