Can someone post step by step guide how to install Debian on new SSD external drive (on windows PC), and set required partitions with Debian built-in tools?

Seems pointless but...
Power off and remove ac plug and any powered peripherals.
Remove power or data from internal windows drive.
Attach external drive to system and power back up to bios.
Be sure external drive shows in bios as a hard drive.
Now install debian from dvd or network or where ever to this drive.
I see no reason to make changes to default but you can if you wish. A ssd has no backup or raid generally so any failure would need backup recovery.
One can make a /home as a partition of they way.
One might need to make /boot on some odd situations. Make it bigger than default.
I'd try to stay away from swap file or swap partition on ssd myself if possible.

The pointless comment is more of a jab about slowing linux down on an external enclosure.

Where user stuff are usually stored? (like programs, scripts, files) Does it make sense to create separate / home partition?

Some folks like to.

Advantages of making more traditional partitions are generally found in recovery and backup and moving. A common home user would find little benefit from traditional methods. I almost never do. I'm too lazy.