Ganesh Visarjan stotrams, mantras, slokas, along with the detailed procedure is given here. There is a proper vidhi to follow on Ganpati Nimajjan day prior to immersion of Vinayaka idol.
There is an Udwasana mantra which is chanted before moving the idol on visarjan day for immersion. Some devotees immerse Ganesh idol on Ganesh Chaturthi itself & the same procedure is followed for these idols too.
Some devotees follow the below mentioned procedure on the following day of Ganesh Chaturthi & immerse the Ganesha idol on Anant Chaturdashi (Usual Ganesh visarjan day).
Offer Lord Ganesha the final prayers – chant aparadha stotram (or else pray to him to forgive for any mistakes you committed during puja). And also pray to him to come early next year so that the celebrations begin again. Visarjan means not offering farewell but sending the Lord to his own abode from where he has come.
Chant the below mentioned Ganesh Udwasan or Visarjan mantra
Yagnena yagna mayajantha devaa:
Thaani dharmaani pradhamaa nyaasan,
theha naakam mahimaana ssanchamthe
yatra poorve saadhyaassamthi devaa:
यग्नेन यग्न मायजनता देवाः
थानी धर्माणी प्रथमा न्यासं,
तेह नाकाम महिमाना ससंचामथे
यात्रा पूर्वे साद्यास्संति देवाः
After chanting this mantra, move Ganesha idol towards Eeshanya disha (North Eastern direction) for a bit.
Then offer Mangal Aarti to Lord Ganpati.
Now, sprinkle some water drops on Ganesha idol & take the Lord in procession (if you are immersing him in your house, just take him to each corner of your home. It is the symbolic way of showing him his house where he stayed during Ganesh Chaturthi utsav). During procession, raise ‘Ganpati Bappa Moriya’ or any other Ganesha slogan you want.
You can also pray at Visarjan place for some time before technical proceedings of immersion take place.