If you are doing the NVQ then you should have been allocated to an advisor/assessor by your training provider, it is his job to help you through.
Writing tech write up's is straight forward. There are templates, but i prefer not to use them with my apprentices.
Firstly you need to be aware that each assessment route summary needs cross referencing to your work 3 times.
You need 3 good examples of tech write up's as a minimum to meet the requirements, usually it takes 6 or 7.
Photographic evidence is also very useful, and so are things like instructions, circuit diagrams, warning signs about PPE, and so on. It is also acceptable to takes notes of any discussions that are held between you and the assessor, this is also acceptable evidence. You should also be observed a few times by your assessor as part of the process.
Writing a tech write up consists of a Before, this is where you discuss the job requirements, what the fault symptoms etc maybe, who the work will affect whilst power is off and so on. Risk assessments, what protective measures you need to take to protect you, other workers, anyone else who maybe in the vacinity, such as barriers, cones, cordons. You need to list the tools and equipment you will use, any permit to work systems in place, and it goes on and on. The more content you get in the easier it will be to cross ref more of the portfoilo.
During, Then look at the job itself, what are you going to do, how will you do it, is anyone helping you, what will you do if you need first aid, step by step explain how you did what you did.
Finally After, clean and tidy up, dispose of rubbish in the appropriate way,de isolate, handback whatever it was you did, to whoever uses it once it is tested and working again.
Your advisor or assessor should explain this to you,
Cheers............Howard