Sponser letter

hi i am student in uk,my husband on dependent visa,i am working 20 hours and my husband 45 hours per week,now we have a little baby 9 monthes old she is born in uk,so now i have to go to college and work,i have need someone who look fter my baby,so i want to sponser my mother in law,plz tell me how can i write sponsership letter to invite my mother in law.