If you are granted the ability to summon something(from a feat, weapon, etc.), then there are 2 ways to do so.
The first is a Summoning Ritual this is an ordinary ritual used to summon something you can summon.
Rite of Summoning
You weave an effigy of hairs and plants and twigs and throw it into the glistening rainbow fire. As it burns up, a servant appears.
Level: 15 Component Cost: 10gp for each level that the summoned creature is
Category: Creation Market Price: 200gp
Time: 5 minutes Skills: Arcana, Building
The chosen summoned creature appears.
The second is a Summoning Circle In a summoning circle, you can simply summon something for free in under a minute. However, you cannot do this during an encounter. Also, a summoning circle can be used at most once per day, and are very rare. They are usually found in dungeons, temples and places of magic.
If one wants to use a Summoning Circle, and is in need of a thing to summon, then one can purchase a Summoning Doll. This is a one-use doll that when used at a Summoning Circle, summons the thing it was intended to look like, and then disintegrates (i.e. a Summoning Doll of a Turtle would summon a turtle)