When you insert a phoneme, if you type an IPA symbol in the IPA Symbol field when the Phonological Features field is empty, then existing phonological features are added by FieldWorks.
Otherwise, you can choose them manually as follows:
In the Navigation Pane, click Grammar, and then click Phonemes.
In the Phonemes pane, click the phoneme. In the Phoneme pane, click the Phonological Features field, and then click the ellipsis button that appears.
The Choose Phonological Features dialog box appears.
If necessary, configure the columns (click ) in the dialog box.
If a feature you need is not shown, click the link to open Phonological Features. There you can insert a phonological feature.
If the feature you need is shown, click it. Then, in the Value column, select +, - or a value you inserted. (<n/a> removes a selection.)
Repeat this step for each feature/value you want to appear in the Phonological Features field.
Click OK.
The feature/value or features/values appear in the Phonological Features field in square brackets, such as [son:+] or [son:+ cons:+].