STEP ONE: pick some calming/peaceful music of your choice
No, for real. I have tried to draw zentangles with different kinds of music and your background noise really REALLY makes a differences. I found I need soft ballads to relax myself into a state where I don't feel the urge to rush - faster songs make me want to match the pace of the music with the movement of my pen and the point of zentangles is to be mindful and precise about your movements, to slow down and relish in the process. This particular zentangle was done with Into the West by Annie Lenox and Only Human by K playing.
STEP TWO: draw a rectangle/square on your paper of choice

I'm using a little notebook to hold a single zentangle. The point is to break up the page, even subtly, so you are not looking at the page like a final piece that has to be perfect and full - lines do not have to be straight to avoid trying to be a perfectionist with rulers too!
STEP THREE: draw a squiggly line

Start from one side of your square/rectangle and wiggle it all the way to the otherside - just go with the flow and don't overthink it!
STEP FOUR: outline your squiggly line starting from the 'top'

I used a different color for the outline color just so I could see where my inspiration started and where it ended up, but you can absolutely use the tool you did for the line. You want to leave about the same amount of space along the entire outline (here is where that mindfulness and precision come in!) and try not to lift your hand from the paper so it is all one fluid line (mistakes are okay!)
STEP FIVE: keep tracing, following only your latest line

When you hit a barrier, like the border, that line stops and you go back to the beginning of your line to keep following that line. You only need to focus on your most recent line until you have completely filled the space above your initial line. I misjudged just how much space was available there in second inward curve and wish I hadn't gone in so deep but oh well!
STEP SIX: start outlining below your initial line

keep following your most recent line and as you do so you may find that you create some negative shapes as lines get too close to one another. That's totally fine and just adds to the uniqueness of your zentangle. If there is enough space within that negative space, you can follow the shape of the lines inside it. If not, then just leave it as is for some extra dimension.
STEP SEVEN: all done!