Need help with Double Jumping

So I am currently working with a platformer and being new to coding I need some help with implementing double jumping into my code. I have seen others asking questions about this aswell but their codes are so different to mine that I have not been able to implement the answers found there. Heres the code I am working with(without any doublejumping failures) :

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour 
	public float speed = 10, jumpVelocity = 10;
	public LayerMask playerMask;
	Transform myTrans, tagGround;
	Rigidbody2D myBody;

	public bool isGrounded = false;

	void Start () 
		myBody = this.rigidbody2D;
		myTrans = this.transform;
		tagGround = GameObject.Find ( + "/tag_ground").transform;

	void FixedUpdate () 
		isGrounded = Physics2D.Linecast (myTrans.position, tagGround.position, playerMask);

		Move (Input.GetAxisRaw("Horizontal"));
		if (Input.GetButtonDown ("Jump"))
		    Jump ();
	public void Move(float horizontalInput)
		Vector2 moveVel = myBody.velocity;
		moveVel.x = horizontalInput * speed;
		myBody.velocity = moveVel;
	public void Jump ()
		if (isGrounded)
		myBody.velocity += jumpVelocity * Vector2.up;

Use OnCollisionEnter2D to verify that the object collided with something. Let the character “always” jump twice. If it collide again enable the jump.