Students often get stressed about learning derivations and what to write, how much to write in a given question. The problem gets aggravated because of so called preparation books which almost write complete chapter as suggested answers. To help you I am giving some suggestions based on my experiences.
Know the Pattern
All of you must have appeared for half yearly and pre-board examinations and most likely you are now aware of the pattern of the paper. To just be sure check the blue print and latest sample paper first. You can check them here.
Identify all the derivations and devices
Just by looking down the syllabus and NCERT books first identify all the derivations and devices (like moving coil galvanometer, cyclotron, transistor as amplifier,etc )working you need to remember. Just make a notes of all these in which you write only that much which is sufficient and can be written within the given time.To help you in how much to write just look at the marking schemes provided on CBSE website where each and every required details are listed out. You can check the suggested marking schemes of previous year papers here.
This will give you tremendous confidence.
Don't write extra
Because of preparing and learning too much students tens to overwrite the answers giving unnecessary details and making answers lengthier and as a result fails to complete the paper. Many of my students have done this fault in past, so please don't write extra other than required.
For 1 mark: just write one word(if that's only asked) or 2-3 lines at max
For 2 marks: 3-4 lines at max
For 3 Marks- not more than half a page unless diagrams are involved.
For 5 Marks: 1 Page around but please check marking schemes to get an idea that what key points must be covered(Better to answer part wise and in points)
All diagrams must be drawn neatly and properly labelled.
For solving numericals please write the steps properly.Try to avoid shortcuts learnt for objective competitive examinations.
Go through Previous year papers
Go through at-least last three year papers as well as last year's compartment paper and try atleat one of them as mock test within time limit.