The Bdim chord is not very nice to our pinky and may take some time to learn. We start with the index finger on the first fret of the E string. Next the middle finger is on the second fret of the C string and the ring finger is on the second fret of the A string. Finally, we have to reach that pinky way over to the fourth fret of the G string. And once you have the pinky in place the other fingers may have moved. Keep your fingers as straight up as possible, especially the pinky. It will take practice and stretching to make that grip and sound all the notes clearly.