The platform is static sensitive - you should make sure that your workplace has a conductive surface that is properly grounded to avoid static voltage build-up. Static discharge while touching components will positively destroy the board.
You need at least a good fine potential-free electronic solder iron. Or in the worst case a gas-powered solderer to do the most of the modifications described here. And at least average solder skills.
However: None of the modifications that require soldering operations are of very high complexity or over-average difficulty range (Except where otherwise noted).
You will need quiet hands nontheless.
Rethink again about the value of the platform. If you are in doubt that your skills will suffice and are a bit worried remembering how much care and money it may cost to get another P-90 platform ... leave it.
Or try to get someone else to do it.
The reworks are not very difficult but before you ruin a good working platform: Think. And think again.
The modifications are mainly easy reversible for the sake of the originalist freaks.
The usual disclaimer: You are doing it on your own risk. The maintainer is not responsible for any damages or losses, neither as direct or indirect consequence.
Or in simple words: if you louse it up - it's your problem !