The 80/20 rule is based on The Pareto Principle which implies that 20% of your efforts will account for 80% of your productivity during any given day. For your to do list, this simply means that 20% of the items on your list will be worth more to complete than the other 80% percent. So each day, you will have to prioritize the top 20% of the tasks and work hard to accomplish those tasks each day.
Take a look at your list each morning. Decide how many items can be labeled most important – meaning tasks, that once accomplished, will result in something more beneficial to you than other items on the list. Is it 2 out of 10? 3 out of 15? Make a notation by the "top" items and focus on completing those tasks each day.
If you finish your 20% and want to move on to other tasks on your list, start the process again so you stay focused and on track.