The simplest way is to take a piece of rigid cardboard and a straight edge. Cut out the shape of your edge and drag it length wise down the edge.

In your case, apply a little more caulk than you need and you would cut out a 90⁰ section of the cardboard (round the corner a bit) and then start at one wall and work your way to the other. This should deliver the results you are looking for.

Edit: you should look for caulk removal tools as well