As the title suggests how do u learn a textbook by urself? Any advice?
I recommend you to read a few pages/topics a day, and complete all the questions at the end of each chapter for each topic. Just read and explore, biology is about exploration, if you want to excel, you must be willing to explore different resources and be willing to learn new things. For example, if your textbook is confusing you, do not stop there, ensure you explore online and search for more explanations, ask forums (like the Biology Units 3/4 Question Thread) and do NOT be afraid to get something wrong! Watch videos on YouTube about the topics at concern, and try to do extra exercises/questions to aid your knowledge, like Biozone or Exam pro!
As for how to learn a textbook by yourself, the one thing I used is reading for meaning, that is, every word used in the textbook is used for a reason. For example, when it says 'cellular aerobic respiration', some books neglect the cellular, but this is important because it describes on the molecular level how cells respire, and not just the organism as a whole. Also, do not be afraid to ask questions, ask, ask, ASK! Do not worry if the question is plain simple, if you never ask, you will never know! Even the smallest spec of detail that you are unsure of, ask!
So in short, dont just read; explore the topics learnt, try to connect everything and I promise you that when you do this, everything will make sense to you, because biology is a subject where almost everything interconnects!