Equip the item you want to charm, click on it; a charm inventory will open.
Drag it next to your main inventory, and drag the charms to the charm slots (ignore the x's over the empty charm slots, it's not like in DD where you have to develop the charm skill).
For charm effects, see
this page.
The A2 summoning doll is near the Spider Queen in the Spider Forest (nr 10 on
this map).
A3 summoning doll is kinda hard to miss, as it will be given to you in the opening sequence for that act.
A4 summoning doll is at the academy level 2 (nr 24 on
this map).