Just tell the client that you anticipate the work taking between X and X hours, and that this would work out at approximately $ XXX.XX to $ XXX.XX (X to X hours times your hourly rate)
I estimate how long it will take me to do a task.
Then I multiply that my by hourly rate.
Then I double that number.
That is my fixed-price quote.
I tell the clients how I calculate this, and I let them choose between an hourly or fixed-price contract.
In the same context, I have been asked to give a quote on a web content writing project.
The heads to be written are unclear.
Also I'm not sure if the quote should be rendered on hourly basis or fixed-price (pardon the naviety but this is my second job on Upwork).
What would you recommend I do?
re: "The heads to be written are unclear. Also I'm not sure if the quote should be rendered on hourly basis or fixed-price..."
If something is unclear, then it is not a fixed-price contract.
You may accept an hourly contract when something is unclear.
Part of the time you will will be working with the client to help clarify what her needs are.
You will bill time while interviewing the client, asking questions, suggesting ways that she can clarify her heads, etc.