As an example, let's say we have a row of cards to interpret: A B C D E F G...
Generally, I would start with A and look to see how it's being modified by B. Then I would look at B to see how it is modified by both A and C, and so on-- but there are no set rules. Pay attention to how the card suits interact. This interaction can guide you through the development of the situation in question. In the end, the manner of combining the cards is always going to be based on the cards that fall, the order in which they fall, and-- most importantly-- your own intuition.
I also tend to read action based on the card order. For example, if we look at card B, card A could be a past influence and C could be a future influence. Card A leads to card B, B leads to C, and so on.
Okay, let's take an actual row of cards to help make this clearer. I am borrowing a 7 card draw that a student was struggling with recently. It's a great example because it illustrates how the cards modify each other based on timing, suit and meaning.
Here are the cards:
The first card is the King of Clubs. We know he is a man, and we could further identify him based on his suit-- friend, colleague, co-worker, ambitious, friendly, goal-oriented, etc.
Face Card + Spade = Unhappy Person
The 5 of Spades modifies the King of Clubs. In other words it tells us more about him. The 5 of Spades represents divorce, feelings of loneliness, being forced to stand alone. So we know that he is either currently going through a divorce or will soon be going through a divorce (if the 5 of Spades had fallen to the left of the King of Clubs, it would have suggested a man already separated or divorced). We can also say he's upset and feeling alone.
Spade + Heart = Need for Emotional Healing
Next we look at the 3 of Hearts which is a very good card. But here we see it hemmed in by spades-- the 5 of Spades on one side, and the 10 of Spades on the other. Furthermore, and it's topped (covered) by double spades-- the 10 of Spades followed by the 4 of Spades. So we can say that although the 3 of Hearts is generally a very good card, in this case the message is not so good. Because the 3 of Hearts modifies the 5 of Spades, it tells us what the man is divorcing himself from. The 3 of Hearts represents a relationship that is ending painfully.
Heart + Spade = Emotional Pain
Now we turn our attention to the 10 of Spades which is a card of fears and worries and suspicions. The 10 of Spades modifies the 3 of Hearts so we can say that the man has worries and suspicions regarding the relationship. The 3 of Hearts can sometimes refer to a love triangle (particularly when hemmed in by Spades), and so we see that the man suspects that his partner has met someone else. We don't know if that is true or not based on these cards, but the suspicion is clear, and the 10 of Spades does indeed suggests secrets.
Spade + Spade = Serious Problems
The 4 of Spades comes next. It represents illness, depression, exhaustion, caution, feeling stuck, etc. Since the 4 of Spades follows the 10 of Spades, we can say that the worries and suspicions may affect the man's health and well being (the 4 of Spades and 10 of Spades falling together is the classic combination for clinical depression). He may feel stuck and sluggish, and unable to let go of the fears and worries.
Spade + Club = Actively Working Through Problems
As we move further into the future we look next to the 5 of Clubs. This card represents active intention, taking action, confidence, etc. Since it modifies the 4 of Spades, we can say that the period of moping and inaction will be short, and the man will be motivated to take action and do something about his situation. We know from his suit that the King of Clubs is generally a 'take action' kind of guy. Also the symbolism of the two 5's indicates taking matters in hand, taking control of the situation, the outcome rests in his own hands.
Club + Diamond = Increase in Power, Improvement, Action Toward Success
Finally we have the Jack of Diamonds which can be read as a young person, or as the man's thoughts, or as advice (the Jacks are messengers). Since this card modifies the 5 of Clubs, which is a communication card, I would read the Jack as advice and say that the man will be actively seeking counsel. Clubs are about communication and learning and progress. Diamonds are often professionals, and represent legal and psychological matters. So the Jack of Diamonds could represent psychological counseling or some kind of therapy.
Diamond = Final Outcome
The final card is the outcome card, and as a Diamond it tends to be positive. So we know that despite all of the bad cards in the row, the man is going to come through the separation fine-- Diamonds represent empowerment, improvement, success, etc. He will take control of the situation and seek the help he needs to move past the challenges.
I think pulling together a long row of cards can be more challenging than reading a spread with positional meanings, such as the Celtic Cross. But it's a skill that can be developed with practice.