Placing stringer is often difficult. Not simply to put it into place, but to keep it there. People tend to hold the pieces with glue. However, the glue burns off before the stringer is anywhere near even tack fusing temperature. This allows the stringer to roll. Also an excess of glue will boil off during the heating and so move the stringer even more than gravity will. Two methods are effective in reducing the ability of the stringer to roll, by flattening one side.
One method used by Bob Letherbarrow is to hold a stringer that is much longer than needed against the glass grinder bit. Use your thumb to hold the stringer against the bit. Slowly pull the stringer between the bit and your thumb or finger, using light pressure, to hold the stringer against the spinning grinder bit. This will slightly flatten one side of the stringer.
Another method is to place the stringers on a prepared kiln shelf, making sure they do not touch. Take the temperature quickly up to a tack fuse, soak for a few minutes and turn off. This will take only an hour or so and enables you to prepare a number of stringers with a minimum of effort.
In both cases, cut the stringer to the length needed and place on the glass with the flat side against the glass. Glue it very lightly to hold it in place while moving the piece to the kiln if it is absolutely necessary. When the glue burns off the stringer should not roll around. It is best, of course and if at all possible to place the stringers in the kiln, so no glue is needed.