Always do one thing at a time. Multi-tasking is unproductive and a myth.
Always know what problem you need to solve and once solved, what measure of success you evaluate your solution by. Then validate your work. Learn from the results, and repeat.
If you are a leader, you have two things to do at the same time. Unfortunate but necessary.
Work on the one problem, your biggest problem, that only you can solve. Know how you will measure your success. Spend time validating the outcome. Know what your key performance indicators are and the problem you are working on should move one of them. If not, you are probably working on the wrong problem. Once you have solved the problem review and learn from what you have done, validate the work against your key performance indicators, and then move to your next problem.
Unfortunately leaders don’t have the ability to solely do this their entire day. They have to first spend time helping others understand what problems they need to solve, how they will measure their success, and once solved work with them to understand the validation results and the lessons learned to be applied to their next problem. They do that first. This is the first priority of their day. They delegate with authority the problems that others will solve with full accountability for the measures of success that they have to deliver. This is what leaders do.
Your worth as a leader is a multiplier of the number of problems to be solved that you can effectively delegate. Probably not a linear line, more than likely exponential.
The rest of your day you work on the one problem that only you can solve.