UICollectionView spacing margins

Setting up insets in Interface Builder like shown in the screenshot below

Will result in something like this:

To add spacing on the entire UICollectionView:

UICollectionViewFlowLayout *flow = (UICollectionViewFlowLayout*) collection.collectionViewLayout;
flow.sectionInset = UIEdgeInsetsMake(topMargin, left, bottom, right);

To play with the spacing between elements of the same row (column if you're scrolling horizontally), and their sizes:

flow.itemSize = ...;
flow.minimumInteritemSpacing = ...;

Swift 4

    let flow = collectionView.collectionViewLayout as! UICollectionViewFlowLayout 
    // If you create collectionView programmatically then just create this flow by UICollectionViewFlowLayout() and init a collectionView by this flow.

    let itemSpacing: CGFloat = 3
    let itemsInOneLine: CGFloat = 3
    flow.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)

    //collectionView.frame.width is the same as  UIScreen.main.bounds.size.width here.
    let width = UIScreen.main.bounds.size.width - itemSpacing * CGFloat(itemsInOneLine - 1) 
    flow.itemSize = CGSize(width: floor(width/itemsInOneLine), height: width/itemsInOneLine)
    flow.minimumInteritemSpacing = 3
    flow.minimumLineSpacing = itemSpacing

EDIT

If you want to change to scrollDirction horizontally:

flow.scrollDirection = .horizontal

NOTE

If you set items in one lines isn't correctly, check if your collection view has paddings. That is:

let width = UIScreen.main.bounds.size.width - itemSpacing * CGFloat(itemsInOneLine - 1)

should be the collectionView width.

You can use the collectionView:layout:insetForSectionAtIndex: method for your UICollectionView or set the sectionInset property of the UICollectionViewFlowLayout object attached to your UICollectionView:

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section{
    return UIEdgeInsetsMake(top, left, bottom, right);
}

or

UICollectionViewFlowLayout *aFlowLayout = [[UICollectionViewFlowLayout alloc] init];
[aFlowLayout setSectionInset:UIEdgeInsetsMake(top, left, bottom, right)];

Updated for Swift 5

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
       return UIEdgeInsets(top: 25, left: 15, bottom: 0, right: 5)
    }