Gather knowledge from different websites and after doing depth research for the topic, you can write for the same topic in your blog too with your own words. But rewrite the same article by changing words and sentences is not good and you should have to suffer from legality.
For you second question,
Technical steps for the same topic remain mostly the same for all articles. So if you think for write the topic which you have suggested here "Installing Snipe-IT on CentOS 7" then also technical steps remain the same which is the legal. But that doesn't mean that you can copy/paste in place of write in your own words.
Content flow is also important to make content more interesting for users. This article will help you to know how to write the article in the proper way.