I understand that it can be difficult to use a textbook chosen by somebody else. We all have our preferences for books and teaching styles and it's not always easy to follow another person's preferences.
I think it's a good idea to do something (at the very least a short activity) from the textbook in every lesson. If you don't do this, students can start wondering why they have the book, especially if they had to pay for it. If you don't use it, students might starting turning up without the book, which makes things difficult when you do want to use it.
You should go through the book and decide how to deal with the activities, there might be some you want to omit. You might feel this is necessary if your students don't need to know the language point or skill. You might think that another activity is necessary because it covers an area where your students are weak. In this case you should find something to replace it with or use the activity but adapted in some way. Some activities might be unsuitable because they don't go into enough depth and they need to be supplemented.
It might be easier to use the skills work in the book. This is also the hardest area to supplement (in my opinion anyway). Use the listening activities and reading passages but adapt them to make them easier for your students. You can do this by giving the students different comprehension questions, by doing more work to engage the students in the topic before they start reading or by pre-teaching and practising vocabulary. For a listening exercise, you can give out the transcript with certain phrases or words blanked out and ask students to fill in the blanks. Before doing this, check out the photocopy rights. You can also adapt the follow-up exercises. Speaking and writing can be made easier by giving more input of language and ideas before the students start. You can also give them a framework for the discussion or piece of writing.