How do I track burn down chart using custom field I created on sub task?

[Sofia Ansari]

so for every US I have multiple Sub task and within each sub task there is two custom fields I have created called Detail Estimate Hrs. and To Do Hrs. I want the Burn down chart to track using the To Do Hrs field. which is remaining work to do for a Sprint.

[Nic Brough -Adaptavist-]

You don't, the burn-down chart measures what you committed to do in the sprint (story/issue completion) not things you are part-way through.  The question "is what you said you would do actually done" is answered with "yes" or "no", the progress on sub-tasks is irrelevant.

For the stories/issues etc (parent issue types), you can use any numeric field for the estimates.  You can select that from the board settings.