In Inkscape (Version 0.92), select the Bezier Tool and put it in BSpline Path mode
Hold down CTRL as you use the tool. This will help you constrain the paths to straight horizontal/vertical/diagonal lines.
This is where you need to place your nodes. You need three nodes like this for each corner.
The Bezier tool will make the curves you need automatically when in BSpline Path mode.
Update June 2020: Since Inkscape 1.0 has now been released, curved corners are available as a path effect called Corners (Fillet/Chamfer)