1) Place glass upside down on top of the middle circle of the sizing guide, centre of the glass (or the stem on stemware) should be over the centre of the guide.
2) Look down on the base of the glass.
3) Note the smallest square on the guide where the largest outside diameter of your glass lines up completely within the square without touching it - this is the number of compartments your rack needs to have.
4) Place the lip of glass against the base line of the vertical guide above with the base/foot of the glass pointing straight up.
5) While holding the glass against the vertical guide, note where the base/foot of the glass lines up on the vertical guide - the next line above the base/foot on the guide indicates the number of extenders your rack will require.