Creating Seamless Pattern

Step 1: Open up your document

• Firstly We need to open a new document with size 300px * 300px.
• We need to create a rectangle on art board with size as same as artboard size.

Step 2: Make and place the elements

• Now we'll need to create a bunch of elements on a rectangle. Make sure the elements which we draw on a rectangle in different sizes.
• By using transforming tool to move or scale your elements.

Image source: http://www.freepik.com/free-vector/colorful-iconic-objects_746429.htm#term=objects&page=1&position=9

Step 3: Create the pattern Swatch 

• Now we want to create a pattern with a transparent background. go to the direct selection tool and select all object which we have created earlier.
• After selected the elements to go to the objects tab < pattern < make and click on it. The pattern is automatically saved in pattern swatch.
• Now you can delete the created pattern on artboard and make a new rectangle and click your pattern swatch.

You've Created your Repeat Seamless pattern.