Note: This tutorial is an English translation tutorial, and the software is not skilled in operation, so try it carefully. If there are any deficiencies in the translation or typos, make up for it yourself.
Step 1
First of all, it's important to set the size of the final mandala before you start creating the brush. The default width of any new brush you create is 1pt. For example, if our pattern is 200 pixels high, applying it to a circle 200 pixels high and wide will create a completely "closed" element, meaning that there is no empty space in the middle.
When creating such a pattern brush, it must be remembered that when applying it to a circle, the top of the brush will be stretched and the bottom will be squeezed. To demonstrate, I created a simple rectangular pattern with 3 circles of the same size placed on top. The middle and bottom of the pattern.
By pre-dewarning elements in the pattern, you can avoid these deformations in the brush application. Basically: if you want to place a circular element in the center of the mandala, the oval in the brush must be an ellipse.
Step 2
The underside of the pattern will be a 100px ×300px rectangle (M) with no fill or stroke. All of the pattern elements behind us must be within this rectangle because it provides the boundaries of the pattern. In View > Outline, you can still View the shape.
Step 3
The first pattern we create will be a simple pattern with a pointed tip. Start by using the Rectangle Tool (M) to create a 100×100px square in the middle of the canvas. Select a bottom corner point and use the Pen Tool (P) to delete it. Select another one, and then use Align Panel to center it on the canvas. Use the Direct Selection tool (A) to select the top row of the triangle and delete it. Duplicate this new path and move it up by 10 pixels, I prefer to do this by holding down Shift and using the arrow keys, by default it will move the path by 10 pixels.
Now, select two paths and connect them to one. Do this twice to close the path. Duplicate the shape (and then use the Transform panel to flip it horizontally.)
We want the shape to be woven. So, we'll be using Pathfinder > Delimiting. This will result in a group with eight shapes. From the right, I selected the three shapes that I wanted to merge that appeared at the top (I selected the bottom chevron, the right side selected Intersect, and the top right corner selected the shapes) and applied the Merge option from the Pathfinder panel. Do the same for three opposing shapes.
The final tentacle of this pattern is to leave it with an "open end", so it will be repeated seamlessly later. Using the Direct Selection tool (A), click on the four end rows and delete them.
Step 4
Another added feature of our patterns is the lines between the different patterns, which will give the mandala more structure.
Simply use the Line Segment Tool (\) to draw a 100px wide line. If you activate the Smart Wizard, you can start the row precisely at the left edge of the "invisible" rectangular shape. Hold down the Shift key while dragging and create a perfect vertical line until you snap to the right edge of the rectangle.
Duplicate this line and move it up 5px. Combine these two lines together and place them above the pointed element we created earlier. Duplicate the group and place it under the Tip Element.
Step 5
The next style will be soft woven waves. The method of creating a V-shaped shape is the same as using a pointed pattern. We won't create it as high, so set it to 30px high.
Select the two left anchor points, and then select Convert Selected Anchor Points to Smooth Out in the Controls window below the Menu and Application Bar. Adjust the anchor point to your liking, dragging the handle until the curve is smooth.
Delete the right anchor point, which should still be a corner. Copy the path and flip it horizontally. Select the two anchor points at the bottom and the connection path.
Duplicate this path and move it up 7px, or as far as you can. Connect the two paths again and close the paths.
Now, the cusp pattern is handled the same way as before: copy shapes, flip horizontally, divide shapes, and merge shapes to create a weave and remove lines between endpoints.
Step 6
Next, we'll create a pattern of circles at the top of the brush. To simplify your workflow, hide elements other than the base rectangle in the Layers panel. Create three circles of 100px diameter each and place them on top of each other so that they align with the top edge of the rectangle.
Duplicate the circles and reduce each circle to a diameter of 90px. Use the Before Minus option of the Pathfinder panel to create a Compound Path for a circle of smaller duplicates.
Duplicate the basic rectangle and place it in front of all the elements. For better visualization, I gave it a red outline. Change its height to 30px. Select Circles and Rectangles, and then use the Pathfinder > the Separate option again. Ungroup the results and remove all shapes except the base rectangle and those with red outlines.
As before, unify the shapes to create a woven look, and remove the lines at the end of each shape.
Step 7
Use the Ellipse Tool (L) to align with the left side of the base rectangle to create a circle. I gave it a diameter of 40px, which is equal to 40% of the width of the rectangle, because I don't want the pattern here to "touch" the bottom on both sides, but leave some space.
Duplicate the circle and change its diameter to 30px, which will create a thickness of 5px. Duplicate these two shapes and move them 50px to the left or 50px of the basic rectangular width.
Since the circles will be at the bottom of the pattern, we will change their height disproportionately, so when applied to the circles as a brush, they will appear as circles and not skewed.
Step 8
To make a simple cross pattern, copy one of the line elements. Using the Direct Selection tool (A), move down the two anchor points on the right. Connect paths to streamline workflows.
Duplicate this shape and flip it horizontally. Similar to the previous ones, we will unify the shape and create a woven look. In the last step, delete the lines at the end of the shape.
Step 9
This is our ultimate model. You can see where I added the horizontal line, and how the elements aligned. For example, at the top, you can see the pattern touching the horizontal line.
At the bottom, I've added a simple horizontal line that doesn't align with our base rectangle, but is 5px away from the edge. A circle will be created in the center of the mandala later.
Step 10
Select an empty rectangle and all styles, and then open the Brushes Panel (F5). Just click on the "New Brush" icon at the bottom and select "Pattern Brush". The default settings will work well with what we want to achieve.
Create a circle using the Ellipse Tool (L) with a diameter of 300px (the height of the pattern brush) and click on our newly created brush and it will be applied to our circle as a perfect, seamless mandala.
Step 11
If you want to continue and turn it into a mandala full of colors, you must enlarge the brush on the circle. Go to the object > extend the appearance.
Simply select a color and hover over the line drawing to see the area that will be filled, then click to apply the fill.
Once all the elements are colored to your liking, expand Dynamic Painting. If you ungroup the results, one group will contain the art lines and the other will have filled shapes. You can edit them as you like, just like you would normally work.
That's it!