I agree with both Deepti and Michael.
The HFSJ chapters on custom tags is about as good as I have seen (or created; I was a course developer for Sun in a previous life).
But nothing compares with learning from experience. I would recommend coding a simple loop custom tag from scratch. For example, have you ever needed to loop over the values of an enumerated type? (to populate the <option>s of a <select> tag for example?)
Imagine your webapp includes an enum of hobbies:
And you want a JSP form to ask the user select their hobbies. You could do this using scriptlet code like this:
Buut as we know "scriptlets are bad, ngKay" (a'la Mr. Garison from South Park).
So a better solution is to use a custom tag:
Your mission --if you choose to accept it-- is to develop the forEnum tag. The easiest solution will be to develop a "Simple Tag" handler. Start there, but when you get that done create a second implementation using a "Classic Tag" handler. When you get that created you will know 80% of what you will need for the SCWCD exam. Note: a "Tag File" is not an appropriate technology for this exercise.
Good luck on your mission and if you need assistance feel free to email me at firstname.lastname@example.org