How far from the end of the piece of wood being drilled should this be positioned, and how can I calculate the required screw length?
The end of the guide is offset from the edge by the distance of the board the holes are being drilled into. For 1/2", then set the guide back from the edge 1/2". 3/4" = 3/4" setback. . Use the Kreg Screw Guide for the length and type of screw to use based on thickness and wood type.
Do some test drill on scrap wood before trying on your finished product. The difficult part is setting the drill bit depth if your kit did not include the bit and collar. Also clamp the two pieces together before screwing the screw in to make sure the two pieces are aligned.
The depth of drill is set by doing this.