To Construct a Circular Fillet

Select the Construct Circular Fillet tool (Drawing > Home > Modify > Fillets split button).
In the Radius field, enter the desired radius of the circular fillet.
Select the first element or segment.
If Truncate is set to First, this element or segment is truncated.
Select the second element or segment.
Enter a data point to accept the fillet and truncation(s), if any.

Construct Circular Fillet. From left: Truncate set to None, Both, and First.