Implement UICollectionView relatived delegate in class.

ViewController.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    @IBOutlet var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.collectionView.dataSource = self
        self.collectionView.delegate = self
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath)
        
        cell.backgroundColor = .red
        
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: 50, height: 50)
    }
}