Returning rental invoice data transfer object When trying to follow single responsibility principle, I often find myself in a position where there is so much to do that I do not know where to start. Maybe there is there a rule of thumb that I could use? 

A concrete example originating from my test project.

    // doing way too many things
    public InvoiceDto GetInvoice(CartDto cart)
    {
        if (cart == null)
            throw new ArgumentNullException(nameof(cart));                                      // validation

        var cartequipments = _carts.GetById(cart.CartId).CartEquipments.ToList();               // quering
        var rentals = cartequipments.Select(o =&gt;                                                // mapping
            new RentalDto                                                                       // object creation
            {
                Name = o.Equipment.EquipmentName,
                Price = _mapPriceCalculatorLogic                                                // buisiness logic
                    .Create(o.Equipment.EquipmentType)
                    .Calculate(o.RentDurationDays)
            }).ToList();
        var loyaltyPoints = _loyaltyPointsService                                               // buisiness logic
            .GetLoyaltyPoints(cartequipments.Select(o =&gt; o.Equipment.EquipmentType)); 
        var total = rentals.Sum(o =&gt; o.Price);                                                  // aggregation

        var invoiceDto = new InvoiceDto                                                         // object creation 
        {
            Title = $"Invoice id : {cart.CartId}",
            Rentals = rentals,
            LoyaltyPoints = loyaltyPoints,
            TotalPrice = total
        };

        return invoiceDto;
    }


My question is how would you clean this up and if possible what would the result look like (helper methods sources not needed).
Make separate methods for getting equipment from the cart, creating rentals from the equipment, getting loyalty points from the equipment, and creating the invoice from the rentals and loyalty points.