First I write a list of all the features that the client wants on their website, then I break down the features into tasks, then I estimate the number of hours required for each task, then I sum all the hours and multiply the final number by 3.

@iamavalos well, I adjust the price and estimated time based on the client's budget. I won't give you a full breakdown because I have better things to do but I will give you an example: you said social network, so it would probably have som kind of chat / direct messaging so I would break it down as follows:
- implementing something that can translate @username into something that the backend can work with - 0.08h
- implementing creating conversations between two people - 0.17h
- emojis - if I can find a plugin probably 0.08h, from scratch 2h
- group chats 0.08h
- group chat moderation 1h
...
- frontend 1h to 4h (depends on the rest of the project)

And then you move to another component of the project, rinse and repeat