Hpnet Important dates tentatively expected to be announced from January 2019 onwards and all details below. 

https://law.careers360.com/articles/hpnet

You can start preparing as exam shall be in May 2019. Make fixed time table and start with fundamental first and revise daily. By December you should have completed enture syllabus and appear for Mocks or sample papers in Janand February and then fully revise again all by March. 

https://law.careers360.com/articles/hpnet-exam-pattern