How will you negotiate salary if the manager has already given you offer letter?

DONT NEGOTIATE AT THE SAME TIME FIRST GET THE OFFER AND JOIN THE JOB THEN PROVE YOUR QUALITIFES AND WORK HARD WITH UNTIRED WORKING THEN U WILL BE ABLE TO NEGOTIATE YOUR SALARY.

There's no point trying to negotiate before you have a job offer; after all, the employer still hasn't even decided if they want to hire you.
And after getting offer letter you become strong.
And for salary range you have to complete your homework like visit the salary websites but sometime they show unrealistic results so its best to contact someone in same field and can suggest you.

Once the offer letter is issued , there is no gain trying to negotiate the salary again! If one is unhappy with the salary offered, only option is to refuse the offer and opt for any other job at hand.
Trying to renegotiate will damage ones reputation or image.
one may be labelled fickle minded !! ability to decide and stick to ones decisions is a desirable quality.
Though salary aspect is important, it is prudent to consider other aspects like, opportunities to learn and develop, experience with a well recognized brand, natural fir to qualifications, previous experience, proximity for daily commuting, chance to live with your family etc etc.
this experience could prove to be a stepping stepping stone for a better job and a senior role.
In the beginning of ones career especially, it is important to join a good company and gain experience , learn and to hone your skills.
In the interest of a long term objective like attaining senior positions like VP, Director, CEO etc, one is expected to plan the career path and be ready if necessary to accept lesser salary and even position.
Even otherwise, ones life situation like caring for elders OR to be able to look after ones children's education etc may require one to find a suitable job, under constraint .
Focus on the ultimate and long term goal of higher achievement , should receive preference over immediate inconveniences.

Well you can always renegotiate any terms of the contracts, as long as you do the tasks you are asked to do initially.
If I am willing to continue with the company, I might give it a try for3 months, after that, any person has the right to discuss his contract again.