The first thing you should do is start reading all the baniya you are able to read fluently, in lardivaar. for example, your nitnem, start reading it in lardivaar and any other baniya you have incorporated into your nitnem. if you want to improve your reading abilities of guru granth sahib jee, i would recommend reading the pad sheh of what you want to practice 10 times, after that, read it again in lardivaar 20 times, then find someone, and read it out to them, with them following in pad sheh, to make sure you retained the right pad sheh. although it is recommended to take official santhiya, as there are a lot of other factors, bishraams, jumkees, bindiya, nasal sounds that also need to corrected if you aspire to read maharaj jee's saroop.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥