On the form tab, next to the configuration textarea, you'll see a small formish looking icon, click that and you are there.
Click image for full size:

I'll go over the properties of each field type later, otherwise the image below might get to a truly large size. So this is the basic interface, with one input field chosen.
Note: one field = one row in the form.

- Move to bottom (Move Up). If this is your top form field, this will move the field to the bottom of the form. If this is a field below the top, then this is a basic "Move Up" button.
- Remove row - delete (Warning, if you have only one row, don't delete. If you do, you can't add any rows/fields in the wizard, that area will remain blank!).
- Move down
- Add row below - in other words, add another field.
- Type - See below
- Label - this is what the user will see, give it something meaningful to you and the person filling out the form
- Required - are you going to require this to be filled in.
- This area will only need to be filled out once.
- Send button label. Now Typo3 won't require this but if you don't put anything there, you'll have a blank button.
- HTML Mode enabled.
- Subject
- Recipient email (add more than one by separating with semicolon).
- Save and leave the wizard open
- Save and exit to the form tab
- Cancel
- refresh the wizard
Once in the form wizard you will be presented with one field (row) and the special configuration for the form.
Although really you can fill this out in any way you wish, I'd suggest adding the special configuration information first, then adding all the fields you want in the form.
Now you can add fields to the form. Links to a discussion of each field type is listed below.
Choose your field type.
Fill in the basic information (usually Label and Required).
Then save it. This step is important as the detail fields will not show up until you save it!
Not all fields have required details that you need to provide, so note each type below to see if there is more that you will have to add to make the field work.
Click image for full size:
Once you have all the fields you wish, click Save and close. This takes you back to the Form content element. Save and close.