Upgrading windows server 2008 to windows server 2019, two step upgrade required

 For upgrading windows 2008 to windows 2019 - it can be straightly done with an in place upgrade to windows server 2019/

To simplify the steps follows:-

1. Upgrade your current windows server 2008 to either windows servre 2012 or 2016 first

2. Then from either 2012 or 2016 upgrade to 2019.

Simple as that.

In most cases there are scenarios where customers with windows server 2008 or 2008 R2 are reaching the maintenance end date, therefore Microsoft gives flexible choices, 

1. Move your Windows 2008 R2 as it is to Microsoft Azure, in which you will be given time of extended warranty, as this gives you time to plan for you future upgrade to Windows server 2019 upgrade. 

You may need to check the latest white papers from Microsoft portal if these features are still available.

Always consider not fogetting ipconfig and systeminfo file to be backup before starting the upgrade.

As for the procedure for upgrading , i would say it's a preety much wizard driven process that you don't have to crack your head so much.

For the steps and screen shots for upgrading, you can already get via google search, youtube for those independent people posting it or get it directly from Microsoft itself since you already would have a runtime license.