Part 1: Making the brim of the hat
1. Select the box tool and press 'n' to enter numeric mode. The dimensions I am using in the picture should be good for a hat that is properly scaled. The box will be fairly flat but wide. Segment the box so that it can be more easily manipulated by the magnet later on.
2. On a new layer, create a disc that intersects the box and is roughly the size and shape that you want the brim to be. Make the disc tall enough to go through both sides of the box. I also added extra sides to the disc so that it will be rounder looking. Where the box and disc intersect is what will become the brim.
3. With the disc on the active layer and the box in the background layer, use boolean (shift+B) and select intersect. You should now have a flat, segmented disc. This is the basic brim. You can clean up by deleting the other layers, which are no longer needed.
Part 2: Making the top of the hat
4. Create a ball on a new layer. It should intersect the brim and be roughly the size and shape you want the top of the hat to be. I also added extra sides abd segments here as well to make sure it would look good in a close-up. Using numeric mode and remembering your size values on this step could be helpful for the next step.
5. Create a slightly smaller ball on a new layer. If you used numeric, you can easily center the ball and subtract a few mm from the dimensions from the last ball's values. This ball will be used later to hollow out the hat.
6. With the bigger ball on the active layer and the brim of the hat in the background layer, use boolean subtract.
7. With the brim and half ball as the active layers and the smaller ball as the background, use boolean subtract again. Now you have made a basic hat shape to work with. You can delete the small ball to clean up.
Part 3: Using subpatch and magnet for details
8. Subpatching requires that all polygons to be subpatched have 4 vertices or less. This hat has many polygons that break that requirement, so now to fix that problem. Activate both layers and make sure nothing is selected. Press 'w' bring the statistics window up. Notice how many polygons have >4 vertices. Click the '-' sign beside ">4 vertices" and it will select all polygons like that. Now use Triple (T) to automatically create triangles in those polygons. All of the problem polygons should now be fixed.
9. With both layers active, turn subpatching on (Tab). Now everything will look smooth, which works well with the magnet tool.
10. The first detail to add to the hat will be the indent in the top. Make the top of the hat the active layer. Select the magnet tool from the modify menu tab or by pressing ':'. Right click and drag to cover the area you want to work on. After making that selection, use the left mouse button to push the top of the hat down. In the picture, you can see what selection area I used and the result. Magnet can be used without selecting an area, but it is more unpredictable.
11. Make some more subtle indents in the top of the hat for more detail. In this picture, I am adding small indents to the front sides of the hat. You can also use magnet to pull parts of the hat out slightly if it still looks too much like a dome.
12. Make the brim the active layer and use the magnet to put some bend in it, using the same techniques as before.
13. If necessary, use magnet, move (t), or other tools to make sure the brim and top of the hat line up properly.
After a few minutes of extra magnet tweaking and a lightwave preset texture, my rendered hat comes out looking like this:
