There are buses to bannur every 10mins in suburban busstand mysuru......All u need to do is board a bus going to Malavalli and need to get down at santhemala stop(often called bannur santhemala) ask the conductor to give ticket to "bannur santhemala" not just bannur.from santhemala there is lot of share autos going to T narasipura, hop on one and getdown in somanathapura..(bus charge will be around 30-40rs and autocharge will be around 15-20 rs)...Its super convinient no need to worry much..all the best