costing and fm will be quite difficult as it is the most unpredictable paper.use a good author s book preferably Padhukas or Ravi.M.Kishore or Saxena&Vashist's book for your preparaions and also take note of concepts ..regarding taxation, read the recent amendments for that ay which is applicable and work out sums and read the provisions from t.n.manoharan or any other good author...regarding accounts for concepts use either ur class notes or any good author's book(preferably R.L.Gupta and radhasamy if u r doing self study) for concepts and work out sums from that book or ur class material.normal hardwork is sufficient for accounts paper.for AS, use D.S.Rawat's book or M.P.Vijaykumar's book...regarding the other subjects yes you can do it by your own.use any good author's book..My recommendation is for audit - pahuka's or surbhi bansal's book...for law- M.P.Vijaykumar's or Munish Bhandari's book and for ITSM - padhuka's or dinesh madan's book.....there is no specific time recommended for preprations as only SMART STUDY is needed..so, study consistently and smartly...Good luck for your preprations and exams
P.S.:Revise the past exam questions for all papers atleast twice before u appear for your main exams conducted by the institute