Making a curved corridor is almost identical to making a normal corridor, so follow that tutorial first HERE.

The only difference when making the curved corridor is that instead of clicking the extrude button you must click the revolve button.

First however you must set the point to revolve around. This is the green pivot point and should be moved to either the left or right of the image. The further out you go the longer the corner will be.

For completeness sake I've set up a map which requires a curved corridor on the right.

Load the 2D editor and create exactly the same shape as in the previous tutorial.

Next we need to move the green pivot point, I've moved it 256 to the right of the shape as in the image on the right. I've circled the green pivot point in purple to make it stand out.

If you then click on the Revolve icon or choose Process->Revolve.

For the options for this I usually choose 16 peices for 360, and as we want a 90 corridor I've set Use to 4. This should give you a lovely 90 degree corridor which can fit beautifully into the map.

You should then be able to move the shape between the 2 archway textures and subtract, creating your corridor. The final product should look something like that on the left.

Trust me, it look sbetter when you play the map....