No Storyboard! I have a collectionView with a diffeable datasource. Only one section. I want to display a header, so I registered a custom cell class:

collectionView.register(SectionHeader.self, forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: SectionHeader.reuseidentifier)

I create a flow Layout as such:

    private func createFlexibleFlowLayout(in view: UIView) -> UICollectionViewFlowLayout {
        let width = view.bounds.width
        let padding: CGFloat = 12
        let minimumItemSpacing: CGFloat = 10
        let availableWidth = width - (padding * 2) - (minimumItemSpacing * 2)

        let itemWidth = traitCollection.horizontalSizeClass == .compact ? availableWidth / 2 : availableWidth / 3

        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.sectionInset = UIEdgeInsets(top: padding, left: padding, bottom: padding, right: padding)
        flowLayout.itemSize = CGSize(width: itemWidth, height: itemWidth + 40)

        flowLayout.sectionHeadersPinToVisibleBounds = true

        return flowLayout
    }

The datasource looks as follows:

    func createDataSource() {
        dataSource = UICollectionViewDiffableDataSource<Section, Project>(collectionView: collectionView) { (collectionView, indexPath, project) in
                return self.configure(NormalProjectCell.self, with: project, for: indexPath)
        }

        dataSource?.supplementaryViewProvider = { (collectionView, kind, indexPath) in
            guard let sectionHeader = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: SectionHeader.reuseidentifier, for: indexPath) as? SectionHeader else {
                fatalError("Could not dequeue sectionHeader: \(SectionHeader.reuseidentifier)")
            }

            sectionHeader.title.text = "Tap and hold to edit an existing Project!"
            sectionHeader.subtitle.text = "tap to view Project notes"
            return sectionHeader
        }
    }

But how and where do I add the header? The 'old way' doesn't work:

override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {(...)}

I'm stumped!

Maybe you are missing the height for that Header?

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        <#code#>
    }

If you haven't already watched my video on this stuff, you might want to consider doing so: https://www.youtube.com/watch?v=SR7DtcT61tA

@twostraws I have! Excellent video. Had some troubles with the sections so I switched back from compositional layout (I had the headers working there) to flow layout. That's where I can't get the header. But I solved it by doing it differently (no header) ...

Thanks for taking the effort to sort this out!