Once you have successfully broken 100, you will naturally set your next goal into breaking 90. In order to successfully break 90, you will firstly need to ensure that your goal is to have no worse than double bogey during your round. The second goal will be to ensure you have no 3 putts during the round of golf. From breaking 90 to breaking 70, the goal will always be to eliminate 3 putts. No matter what skill you have as a golfer, no 3 putts is always very attainable.
The next task is to ensure your preparation is always the same coming into every game. At the minimum I would ensure you get to the range at least once to twice a week and arrive for your golf day an hour before to allow sufficient time to warm-up. During your warm-up, ensure you go over your pre-shot routine and alignment and take roughly the same amount of time for every shot. You can have one to two swing thoughts max but the goal is put your mind and body into the same state every time over the ball during your warm-up so that that you can simulate exactly what the game will be like.
You will need to ensure that your game strategy is much improved. For eg. knowing when to hit driver and when to hit iron off the tee for safety. The goal is put yourself in the best possible position on each hole without taking massive amounts of risk. When in trouble off the fairway, take more the conservative approach rather than the risky play. This means when hitting out of trouble, you could most likely be within 100m of the hole for the next shot. Ensure during your practice that you focus on hitting different distances so that you become very proficient from any range.
Lastly you need to be quite sound with different shots around the green for eg. how to hit a high shot over a bunker and getting out of the bunker in one shot. If you can do this consistently you will no doubt be able to remove those big scores off the card.
Just remember when trying to break 90, try and not focus on the score so much but eliminating as many unforced errors as possible.
If you'd like to learn more just contact myself for a lesson and we can definitely reach your goal of breaking 90!