I am about to fill the tax returns application. Do I need to fill in the squares my informations p 1,2,3, 4 or I just need to sign as specify? What about the sending, as I don't have to pay any stamp, do I need a special envelop?

If you can just follow the instructions on the first page by signing the pages where indicated with the 'X' and completing the employment history page in full.

If you are sending from the UK you do not need any stamp. Just put the application in an envelope and send it to:

Freepost RLZA-XGXA-LAAB
Castlewood House
77/91 New Oxford Street
London
WC1A1DG