The Installation Stage of the Cyber Kill Chain is when the threat actor establishes a foothold in the target's infrastructure. The victim of the attack should start to notice signs of an attack if they are monitoring.
Weaponization in the cyber kill chain uses the intel the threat actor obtained from the reconnaissance stage to create a suitable piece of malware to infect their target.
The Reconnaissance Stage of the Cyber Kill Chain is where the threat actor probes their target as thoroughly as he/she can so that the threat actor can gather as much intel as he/she can.