Firstly, if you put the fabric on the bias binding (bought bias binding here) past the centre to touch the binding edge on the other side like it shows above.
Then start sewing your bias about 1 cm before the attaching fabric and with a bit of a tail... I find this helps place the bias in the right spot and stops that fabric down the hole problem. If you notice on the left to the photo (unfortunately upside down and unable to change??) the bottom part of the bias extends a bit further on the underside of the fabric and so if you concentrate on getting the top edge nice and straight, the bottom catches automatically.
This is how it looks on the top and the underneath. All caught up. So that's my