In short

In the KDevelop menu File->New, select a Widget (.ui) file and give it a name (kprojectdlg for that example), check add to Project. Click OK.

Qt Designer starts. Work on your form in Qt Designer then save your file and close designer. The designer file appears then in KDevelop under User Interface (kprojectdlg.ui).

Create a new class (KProject as name for example) (Project menu then New class...) that inherits from your dialog file (kprojectdlg) and is a QWidget-childclass.

Add your slots and others signals or member functions in the inherited class KProject.