This is going to be difficult.
What you're looking to do goes against the very grain of what everyone does to lead highly productive lives. You are going to shun multithreading your mind.
For that step ONE is to stop multi tasking. Consciously do just one thing at a time.
eg.
Don't watch a video when you eat. Just eat.
When you start doing this, it'll screw your mind. You start getting withdrawal symptoms because habits built over the years fuel your serotonin receptors the same way everyday.
Stick to this unbundling of tasks for atleast 3 weeks. Through the pain. You'll be in a place to start undoing the multithreading.
Step TWO is to supplant step ONE by putting away distractions. Some you may need to give away / discard because control is difficult.
eg.
When you are out with someone don't get your phone out of your bag.
Do these for three months before you start working on being actually mindful like what the masters recommend.