There are a few things to consider.
1. Frequency
Skills need frequent practice. Three times a week is a good starting point.
2. Success
Progress by breaking the movement into smaller pieces. Leaving the session with a happy face helps the skill stick: you learn what to do instead of failing over and over.
With that in place, a few things are specific to the double-under.
3. Achilles tendon power
If you have not been jumping or running on the front foot, chances are your Achilles tendon is a bit weak. The jump comes from the tendon and the soleus. They work together and need to be strengthened.
4. Rhythm
Jumping is a repeated motion, so it has a rhythm. Everyone has their own, set by height, weight, timing, tendon power and more. Let's skip the technical part and cut to the chase: double-unders are about BPM, beats per minute. Find your BPM while jumping and you unlock the skill.
5. Timing and coordination
Jumping and moving the hands at the same time is hard. Break it into smaller pieces that are easier to manage.
One example is skipping with a double clap. Managing that gets you really close to the double-under, but you obviously need to practise with a rope too.