After BCS prelim exam you will get maximum 2 months time. You have to prepare for the written exam within 2 months. Some tricks will take less time.

1) Solve previous year question first. An idea will be born.
What kind of questions come up?
What topic does it come from?
You will understand them.

2) Arts and commerce students are afraid of science. There is nothing to fear. Questions will be on general topics. You don't need to read much about the numbers of substances, chemical reactions. Check 2-1 reactions according to the topic .

3) Figures, reactions (if appropriate), tables etc. should be given in science. Giving these will get good marks. Short questions will come in science. You will get many questions of 1 or 2 marks. They do not require much writing . Write only what you want.

Watch this video on BCS Written Preparation (Science).
BCS Written Preparation (Science) 4) The computer part has some information to remember. For example: What is BIOS? What is DBMS? There are several terms like this. They need to know their full form and association. Give the information in the form of a table with a pencil on the computer. Assume the classification is original. of input and output devices. You will get more marks if you put it nicely in the form of a table . Watch this video on BCS Written Preparation (Computer and Information Technology). BCS Written Preparation (Computer and Information Technology)

5) Examiner prefers to read table rather than paragraph. Consider the comfort in the eyes of his examiner. The better your presentation, the higher the marks.

6) Give some time for the electronics part. There are several new topics here. Which you don't already know. So read more.

Search Wikipedia for each topic. You will find everything in the wiki.
Then by searching on Google, click on the image button. If you put it in the image, it will show the picture about that subject. If you see the picture, you will remember it.  And the picture will be drawn in the examination hall.

Finally to get marks in science and computer you will need:
1) accurate information,
2) And beautiful presentation.
Then you will get more marks. Aim for 65-70 marks in this section. BCS Written Preparation: General Science Booklist: 1) Oracle Science Guide. There is no comparison to this book. You will not find a better book in the market. All subjects of science are well analyzed. Read this book alone.

2) 8th, 9-10th and higher secondary books should be read. All syllabus topics are taken from these classes. So read each topic from these books.
3) Previous Year Question Solution Guide. You can take any one.
BCS Written Preparation: Computer and Information Technology Booklist

1) Guide to Easy Publishing. This book is better than other books. Or you can buy books from professors.
2) Higher Secondary Board Book.  Buy books by two authors. Mahbubur Rahman Sir and Mujibur Rahman Sir. 

So far today with BCS written preparation (General Science and Computer & Information Technology). thank you