Tajweed and its importance can only be studied with a teacher. Online Quran Teachers are the best at teaching Quran with tajweed.
The same rules can be studied independently, but their correct use can only be achieved by hearing, chanting and being corrected by a qualified Quran teacher.
In the days of the Prophet (Peace be Up on Him), there was no need to study tajweed because they spoke with tajweed.
Online Quran Teachers will teach you based on repetition, the student and the teacher will work together on the pronunciation of the letters obtaining a description of the points of connection.