The first thing to know is that there is no one "best system" for ear training. You can find plenty of approaches, some of which are brand new and unproven, others which go back hundreds of years, but every musician comes to ear training with a different background (your instrument, your level of theory knowledge, your genre preferences, etc.) and different goals (to play by ear, to improvise, to be more expressive, to compose and write songs, etc.).
That means that although there are some techniques that are very effective for a broad range of musicians, they must match your own musical life if they're to help you make consistent satisfying progress towards your goals.
For example, a singer-songwriter might want to focus on chord progression ear training, while a drummer might be entirely motivated by rhythm ear training. A jazz saxophonist might want to master intervals for improvisation before exploring other topics.
Even the best ear training resources will be a waste of time, money and effort if they aren't well suited to you or you aren't able to get support and guidance to use them effectively. This is what leads most musicians to "dabble" in ear training and get bored or frustrated after a few weeks.
The second (and perhaps even more important) thing to understand about ear training is that it is almost never a straight-line journey. This is why most musicians really struggle with their ear training course at music college or conservatories: a single linear course is simply not a good way to learn this kind of skill. It requires adaptability. You need to spot when something isn't working well and be able to adjust your course to keep moving forwards.
The musicians who succeed with ear training aren't ones who find "the perfect method". They're the ones who are committed to the journey and willing to use a variety of resources, adjust their course and seek help along the way.
So that's the mindset to approach ear training with. Yes, you should try to find the best resources, and yes you should be persistent with each plan you set. But don't expect to find a magic bullet that will make everything easy, smooth and fast. Our ears just don't learn that way!
Once you understand those two things, ear training becomes easier. You are clear on your background and goals so you can choose training resources (apps, MP3 training tracks, courses, etc.) which match them well, and you know that when you get stuck along the way you can seek help and adjust your course to keep moving forwards.
Now let's get specific.
The best way to practice ear training is... regularly! With aural skills development "a little and often" is much more effective than occasional mammoth training sessions. You should plan on spending 10-15 minutes per day, pretty much every day. For a single skill (such as recognising intervals, scale degrees or chord progressions by ear) this amount of training on a daily basis will let you make steady steps forward and see clear progress.
That time should be spent on "active" training: meaning you are giving your full attention to doing exercises where you are training and testing your ability to recognise musical elements by ear. For example, practising with an interactive ear training app.
If you have more time available, great! But keep in mind that the ears fatigue, and when you find yourself struggling it's time to take a break - or at least change topic for a while.
As well as that you should consider "passive" training where you aren't taking any action (like answering questions) but you are paying attention to music in a specific way. The most important activity of this type is called "active listening" where you are listening to music as carefully as you can and trying to do certain specific things, such as following the bassline or identifying all the instruments or effects being used. This kind of passive training can easily be done throughout your day, any time you hear music, and so many musicians find it an effective addition to their core "active" training.