Usually an applicator is available with the help of which you can put it in deep.

Its available with pessary

Pessary can be inserted with a finger or with an applicator. In either case you have to lie down in the bed, flex your hip joints and knee joints so that your feet are resting on the bed and both the joints are flexed. Hold the pessary between thumb and index finger and place it in the vagina. With your index finger push it into the vagina as far as you can. Normally length of the vagina is  9-10 cm. Length of the finger is also similar. Some pessaries come with an applicator. In that case the pessary is kept at open end of applicator. The other end has a plunger. After inserting it into the vagina (as much as you can) , push the plunger. Pessary will be put in proper place. The name of the instrument is pessary applicator.