For those having trouble controlling flex nibs for the first time, I have a small suggestion which may help.
The humble pencil can be a very useful aid when starting out with flex writing and you can quickly become accustomed to the feeling of writing with a nib producing thick and thin strokes by pressure.
Practicing with a soft pencil (I use a 6B) feels very similar to using a pen with a flex nib, but there is no nib to dig into the paper - a common problem when beginning.
I used this technique when starting out, and quickly graduated to flex nibs in dip pens. It's not an end in itself, but it can be a useful stepping stone to developing a feeling for this type of script. Just remember that all upward strokes are as light as possible and all downward strokes swell with controlled pressure. To achieve a stroke difference with a pencil. it may be necessary to blunt the point, slightly.
There's no mess, and it's good fun - try it!
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caliken