You must have two surfaces that intersect to create the fillet between.
Use the Surface wizard or select Fillet surface from the surface toolbar flyout.
Name the surface for future reference
Click Pick and select surface 1, or select the surface from the dropdown list box.
If you want to trim surface 1 with the fillet, set the Trim to fillet check box. Set the Trim Tolerance to affect the accuracy of your trimming. It is recommended for most parts (less than 1 foot cubed) that you set this between 0.001 and 0.0001 inches (0.0254 to 0.00254 mm).
Click Pick and select surface 2, or select the surface from the dropdown list box.
If you want to trim surface 2 with the fillet, set the Trim to fillet check box. (See step 5 regarding trim tolerance.)
You may also set an Intersection to limit the range of the fillet, or to clarify which intersection of the surfaces to consider. Intersection places the fillet along the curve described by the intersection of the surfaces. This curve must actually exist in the drawing.
Set the Arcs only construction switch box if you want to create just the arcs that would comprise the fillet. You could then combine these arc curves to create other surfaces.
Choose the Corner for the fillet. Corner refers to the drawing in the dialog box to help you specify which of the four possible sides of the intersection you want the fillet to be in.
Set Tolerance for how much the resulting surface may deviate from the exact fillet. It will vary by no more than value you enter. If you want high quality fillets for parts that are approximately 1 foot cubed, the recommended settings are between 0.001 and 0.0001 (or 0.0254 and 0.00254 mm).
Define the Arc step of the fillet. If you select Variable, the spacing of the fillet cross sections will be calculated automatically based on the Tolerance. More cross sections will be placed in curved regions of the fillet as in this figure .
If you uncheck the Variable button, you must set an Arc Step. The spacing will then be constant between cross sections as in this figure .
Set the Begin radius. Having two settings for radius lets you create a fillet that grows or shrinks along the path of the fillet as needed. For a closed fillet, the Begin and End radius need to be the same.
Set the End radius of the fillet as applicable.
Click Finish or OK to create the filleted surface.