This a very helpful video. I just got a new P line and gear 1 was jumping. Rather than take it back to the store I followed these steps to fix it.

@Brompton Bicycle seems like after doing this, the gears got noisier and shift less smooth. Also, during the fold, the chain gets stiff and don't move with the pedal in the kept-position. It sort of get bounced up and have a portion of the chain slacked off...

What are the tools required to do this?

0:39 Before cable tension by screwing the barrel adjust fully in, what gear shifter i should set? gear 1 gear 4 ? Please make a clear.

good!

You should rename this video 'Brompton Derailleur Gearing Adjusment' so that it is easy to find

The trick for me was that at the last step of the derailleur set up I had to manually go to gear 3 and keep it there with my hand before adjusting the cable. Just pulling the cable out was not enough - there was not remotely enough tension to even go to gear 2. I believe the video could be clearer there, it took me quite some time to figure out the issue.