How to design a repeated pattern in illustrator

First of all, you have to design some elements that you like. In my artwork, I have design some floral and leaf elements.
Screen Shot 2017-07-19 at 11.56.33 pm.png
Second, you could duplicate the elements in the artwork and expand all the elements in the artwork

Third, you could add a square on top of the artwork.
Screen Shot 2017-07-20 at 12.01.13 am.png
Forth, you could select all the elements.

Fifth, you have to tick the object>clipping mask>make and tick Pathfinder>Crop.

Finally, a vector pattern is done.
Screen Shot 2017-07-20 at 12.14.09 am.png
After the completion of vector pattern, you easily make the pattern as you like.

Please take a look at my shop !There is a lot of illustration and vector patterns for you to explore!
https://www.shutterstock.com/g/cutequokkahappiness
Screen Shot 2017-07-20 at 12.17.35 am.png