Collection View Layout Problems

I'm working with CollectionViews and currently have the problem that everytime I implement a UICollectionViewFlowLayout my original layout gets "destroyed". If I dont implement a FlowLayout, my CollectionView is just one row that has the length of all items in it and everything works fine. But the elements in the CollectionView are spaced really far from another. To use a smaller spacing I need to implement the FlowLayout, but as I do that, my CollectionView uses a grid layout AND each element in it shrinks down to like 20x20 pixels.

Where do I tell the FlowLayout to use just one row and how can I lower the spacing between elements in a CollectionView? I saw something like

ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical)
    {
        ItemSpacing = 20
    }

but I dont know where to put that code. Can someone help me? :/

Thats what my current FlowLayout looks like:

    public class TestFlowLayout : UICollectionViewFlowLayout
{
    public TestFlowLayout()
    {
    }

    public override bool ShouldInvalidateLayoutForBoundsChange(CGRect newBounds)
    {
        return true;
    }

    public override UICollectionViewLayoutAttributes LayoutAttributesForItem(NSIndexPath indexPath)
    {
        return base.LayoutAttributesForItem(indexPath);
    }

    public override UICollectionViewLayoutAttributes[] LayoutAttributesForElementsInRect(CGRect rect)
    {
        return base.LayoutAttributesForElementsInRect(rect);
    }

    public override void PrepareLayout()
    {
        base.PrepareLayout();
    }

    public override void InvalidateLayout()
    {
        base.InvalidateLayout();
    }
}

I also tried something like this in my ViewDidLoad:

CollectionView.CollectionViewLayout = new TestFlowLayout
        {
            MinimumLineSpacing = 0,
            MinimumInteritemSpacing = 0,
            SectionInset = new UIEdgeInsets(0, 0, 0, 0),
            ScrollDirection = UICollectionViewScrollDirection.Horizontal
        };

but I still get a grid layout instead of a linear one.

I also added

public override nint NumberOfSections(UICollectionView collectionView)
    {
        return 1;
    }

in my UICollectionViewDataSource. But nothing works..