Open a new 600x600 px document.

We will import here a symmetrical object that we already rotated at 45 degrees.

Step 2.

We will now focus on the upper right corner.

We bring the object at x=600 and y=0. As we can see, the diagonal is not complete. In order to create a continuous pattern, the object that we want to repeat must occupy the entire diagonal (x=600, y=0 / x=0, y=600). And to do this, first of all we will cut the vectorial element.

Step 3.

Use Direct Selection tool (A) to find the coordinates of the points from where you should cut. In this case we have x=530 and y=70.

Step 4.

Place the whole object at the following coordinates: x=670, y= - 70.

Step 5.

We will now move to the bottom of the diagonal. We will use the Direct Selection Tool to find out the coordinates of the points where we’re going to cut over here. Those will be x=180 and y=420.

Step 6.

We will now create two objects which will serve us as cutters. We will create a square and we will place it at x=180 and y=420 for the left down corner and another one that will cover everything that comes out of the working space for the upper right corner.

Step 7.

We group the two objects that are used to cut. We ungroup the element that is going to cut.

Step 8.

We’re going to use MINUS FRONT from PATHFINDER for all the five color groups.

The point with coordinates x=180 and y=420 will become x=0 and y=600, but not in this step.

Step 9.

Now , our vector image it looks like this.

We’re going to copy the entire vector element in the same place (CTRL+F). We will name it ''FIRST'' and we will lock it from the Layer panel.

Step 10.

We’re going to cut the left side until x=180 and the bottom part that is exceeding y=420.

Make a shape and place it until x=0-180 and another until y=420-600. Unite its from Pathfinder.

Ungroup our vector element. There will be 3 groups that intersects the shapes created. Select the shape and copy it ( Ctrl+C ), hold SHIFT and select one group from the three mentioned. With the ALT taste press MINUS FRONT from PATHFINDER. After this , CTRL+F and select another group ... to the end.

After this , make a group with our cut and chipped diagonal vector element. Lock it and and unlock the one called FIRST.

Step 11.

From the ''FIRST'' element we will cut the opposite step 10 in the same way.

Step 12.

It will result two elements. Group them each one separately. We’ll name the small left item ''A'' and the right small item ''B''.

Step 13.

Unlock the third vector elements from Layer palet.

Move A+B into the third’s square element, and then we will group everything. We are doing this because we want to resize all the 3 elements while preserving the proportions between them.

Step 14.

We resize the elements paying attention to:

a. The REFERENCE POINT is the right upper one;

b. CONSTRAIN WIDTH AND HIGHT PROPORTIONS is locked;

c. We will type 600 instead of 420 in WIDTH or HIGHT field.

Step 15.

Ungroup and get the two smaller elements ''A'' and ''B'' into the corners: the element ''A'' to down right corner and the element ''B'' to upper left one. At this point, our pattern will look like in the next picture. But we want to add one more diagonal.

Step 16.

We copy the big element twice (CTRL+C/CTRL+F) and move them to x= -300 and x=300.

Step 17.

We cut everything that exceeds our artboard.

Step 18.

We copy the element ''A'' (CTRL+C and CTRL+F) to x=300 with Right Reference Point.

Step 19.

We copy the element ''B'' to x=300 with Left Reference Point.

Step 20.

Select all > Edit > Define pattern.

Enjoy!