There are several of these floating around, all of them quality and well worth checking out. This one is basic.
1) Open a New Image of any size you like. The size is dependent upon the size of the result you desire. For the tutorial I used a size of 1000 by 1000 pixels at 300 DPI resolution.
2) View / Show Grid. View / Snap To Grid. Image / Configure Grid (set to increments based upon desired result.) The tutorial uses a Spacing of 50 pixels for both Width and Height.
3) New Layer. Name it "Square." Rectangle Select Tool. Tool Options / Expand From Center + Fixed Aspect Ratio selected. Draw a square of desired size. Bucket Fill tool. Tool Options / FG Color Fill / Fill Whole Selection.
4) New Layer. Name it "Circle." Rectangle Select Tool. Tool Options / Expand From Center + Fixed Aspect Ratio selected. Draw a circle of the same size as your square. Bucket Fill tool. Tool Options / FG Color Fill / Fill Whole Selection.
5) Duplicate your Circle layer. Move Tool. Tool Options / Move The Active Layer. Move your circle layer up exactly one-half of the circle diameter. If your circle is 500 pixels, as it is in the tutorial, then move it up 250 pixels.
6) Move your circle2 layer left exactly one-half of the circle diameter. If your circle is 500 pixels, as it is in the tutorial, then move it up 250 pixels.
7) Merge Down your circle 2, circle 1, and square layers.
8) Rotate Tool. Rotate the merged layer 45 degrees.
9) Layer / Autocrop Layer.
10) Alignment Tool. Tool Options / Relative To Image. Align Center Of Target. Align Middle Of Target.
This works with any size image.