To be honest, I don't remember how I did it. It was right after I first started playing with 3D files and I was experimenting with everything. However, while I don't think this is how I did it, here is a method that I think will work:

1. The Santa hat is Thing 5330.

2. Open your head file and the hat file in netfabb studio. Position head with flat back exactly on the platform

3. Scale, drag and position the hat on your head exactly as you want it to print.

4. Execute a cut on the Z axis at 0.00 mm. This will cut off the back of the hat even with the flat back of the head. Remove the new part that is the back of the hat.

5. Without moving anything, export each part as an .stl.

6. Open both parts in MeshLab. They should be positioned exactly as you had them in netfabb. Without moving anything, -->Filters-->Mesh Layer-->Flatten Visible Layers. This should join the parts.

7. Export Mesh as a new .stl, open in your slicer and slice to print.

Let me know if it works.