How do I find the ID of a custom field, contribution, option value, etc.?
The easiest way is to hover your mouse cursor over a link that refers to the item in question.  In the link that apppears in the bottom of the browser, look for the part that says &amp;id= and note the number after it.

For instance, in the first screenshot, I hover over the "View" link to see that the contribution's ID is 45:


In this example, the custom field ID is 3:


In a few cases, there won't be any &amp;id=.  For instance, with groups it will be &amp;gid=.  They should all have "id" in them though!  Just make sure you don't pick the wrong one - e.g. in the first screenshot above, "cid" is the contact ID, not the contribution ID.