Every action in our system -- petitions, events, forms, fundraisers, ticketed events, and letter campaigns -- has a drag and drop form builder you can use to manage default form fields such as first name or address and add extra form fields to your action. Click the blue "Edit Form" button to open the form builder. You can learn more about the form builder here.
The pre-built questions section in the upper right of the form builder will contain the default fields (such as first name) as well as questions you or your group has created that have not been hidden. Questions are attached to groups, so if you don't see the questions you expect, make sure you have made your action sponsored by your group then try looking at the form builder again. Choose a question from the dropdown and then drag it into your form on the left side. Click each question to set whether the question is required or not. If it's a checkbox, you can also choose to have it pre-checked. Click and drag each question to re-order. Click and drag and move the question out of the form section on the left to remove it from your form. Some questions, such as email address and country, will not be able to be removed or made non-required.
You can also make conditional questions, where the question will appear only if another question has a certain answer. Put the name of the custom field (the 'name' section) in the 'show if the following field name' input, and the value you want it to have in order to show this question. By default, the question will not show up unless that field has that specific value. You can use the wildcard symbol "%" to capture any value.
For example, you could have a question with the name "childcare" which asks "do you need childcare?". If the answer selected is "yes", then you want a question named "childcare_number" to ask "how many children do you have?". When you're creating the if/then question for "childcare_number", put "childcare" in the "show if the following field name" input. Put "yes" in the value.
If you have the correct permissions, you can add ad-hoc questions too in the form builder, by dragging blank questions from the left and adding names, labels and whatnot by clicking on them when they're in your form on the right.
When you're done, click the green "Save Form" button to preview your work on the creation page, and save your page to save your form.