I use literally just Notepad, because it's always available on every Windows desktop and server. On my laptop I use a decent text editor, but trust me, Notepad is sufficient.
I have a simple style that is easy to fill out:
[.] Commenced high-level task.
[o] Completed sub-task
[.] Incomplete sub-task
[ ] Not yet started
[-] Cancelled or not-applicable
[?] May not need doing at all, or pending advice
[!] Issue or error
[ ] Include command-lines under the tasks:
wget https://test.com/foo
[o] Record output after the tasks if needed -- "Task ID: 929123"
Of course, the aim is to slowly convert something like this into a script or a service desk workflow.
In my experience, it's almost impossible to correctly automate a complex, high-level task given no other information that its English description, such as "build a new web server for a developer". However, if you've built 5-10 such servers manually, diligently recording the tasks in a checklist each time, then the script practically "jumps out at you"...
As a practical example, most engineers will say that to install a new physical server (not including the OS) is about 5-10 steps. I counted 50-100 after developing a checklist!
E.g.: Did you remember to:
Check that the firmware version matches the version of the other servers in the same cluster?
If not, did you update it?
Check the NIC firmware too?
RAID firmware?
System time, date, and time zone?
Secure erase / factory reset the BIOS?
Upload an SSL certificate for the management port?
Register it in DNS?
Register it in the CMDB?
Register it in system-management software?
Burn-in test the memory?
Configure LACP on the NIC?
On the switch too?
I have personally witnessed major issues, even outages, caused by the above being skipped by careless engineers.
"Oops."