There are multiple approaches to being focused and you should find the one that works for you depending on how you usually organize your activities. If you use to-do lists, you may put every task associated with the main activity to the top. In case you prefer clean schedules, you may choose specific periods of the day to work on it. Or if it's easier for you to think in terms of hours, choose the amount of time you want to spend on the selected activity daily, weekly or even monthly.
Since focusing doesn't mean you stop doing other activities, what changes then? If you decide to focus on something, it simply becomes the top priority for you. That means that you can't compromise on this thing, you just have to do it. Other activities become optional. If you have time for them — great, have fun. But if not, keep having fun doing what you love and what you decided to focus on.
But what if you love doing multiple activities that are not connected to each other? Can you focus on multiple activities? The answer is: maybe. Imagine you focusing on one activity. That means it's 100% priority for you and you spend 100% of time that you want on it. If you choose to focus on two things, the time will be divided between them in some way. 50/50 or 60/40 probably, simply because 90/10 means you are probably not focusing on the second thing anymore. And since you have a limited amount of time, unfortunately, you will achieve the results in both of these fields twice as slow as if you chose only one. Now imagine focusing on three, four, or even eight things. Does slow progress work for you? If yes, you surely can focus on them all. But if you want to achieve things faster — you need to make a choice.