You can use FieldIt from any document in Google Documents. All the functions are availble under the "Extensions > FieldIt" menu once you have installed the add-on.
The remainder of this page will guide you step by step to create your first document using FieldIt. You can also check-out a full example document here.
You can add text field in your documents, that will be replaced by a text string. For instance, you can add a fields for the first and last names of the recipient of your document. For this purpose, place the cursor at the location of your document where you would like to inser the fields, and select the menu entry "Extensions > FieldIt > Insert variable...". A dialog like the following one should pop-up:
Fill the dialog as shown in the screenshot and click "OK". Then you can repeat the operation to insert a last_name variable. Your document should now look as follows:
If you change your mind about the settings of one of the variable you just created, you can select "Extensions > FieldIt > Variables..." in the menu. A new dialog opens. You can select the variable you would like to edit, and for instance check the box "required". This ensures that the user will have to fill that field before completing the document.
FieldIt also supports boolean and enum variables with conditional "if ... then ... else ..." blocks. For instance, you can create an enum variable named "role" whose value can be "student", "parent" or "teacher". For this purpose, select again "Extensions > FieldIt > Insert variable..." in the menu, and fill the dialog as follows:
You can get a text like that in your document:
Boolean variables work similarly.
In general, you never want to fill the master copy of a document template, but rather fill copies of it. To prevent filling the document by mistake, you can lock it. For this purpose, select "Extensions > FieldIt > Document properties..." in the menu, and check the box "Require copy before evaluating this document" in the dialog:
To complete the process, you can now create a copy of your document, and select the menu entry "Extensions > FieldIt > Fill document...". A dialog opens to ask you filling all the fields of the document:
Once you click OK, your document will automatically get filled: