I remember you were already doing diagrams.
I recommend the existing tutorials, they are old but for me they are still valid, I learned there and then with practice they achieve good results.
Http://skyscraperpage.com/diagrams/help/
Here you can see an example of two diagrams, from 2014 to 2016
Http://skyscraperpage.com/diagrams/?buildingID=98843
From programs, I start with PaintShop Pro and now I use Adobe Photoshop CC