The way I write, I start out with the kind of object I want, and then come up with a narrative. I realize that this is a bit unorthodox and the opposite of the usual advice given, but it seems to work for me at least.
Once you have both of these things, you can begin. Start with the Description: run through every one of its anomalous properties briefly and promptly, ordering them so that each one will segue into the next one as nicely as possible. You can also put the discovery here, but I tend to use the discovery as part of the addendum story instead.
Now then. In the addendum is where we reveal what the SCP means. This is where you will subtly weave in hints to the narrative or background of the object.
See, a lot of my works follow the general "description -> addendum that gives it meaning". People will often repeat the catchphrase that there needs to be a story. This is partially true. I've found that implication alone goes a long way. To a certain extent, the less you explicitly say, the more your readers piece together on their own, making for a more satisfying experience. To a point, of course; too little information and there's no clues to go on. With experience, you'll find a threshold that both you and your readers will find comfortable.
Note: You will struggle with "is it enough?", at least in the beginning. If you're not used to this style it's gonna take some 'feeling your way'. Even now there are SCPs of mine that have been described as too sparse, but in the end, if the SCP remains positive, only you can decide exactly where to draw that line.
Consider this example: you're in the late 1800s, writing an SCP about a crashed fighter jet. Instead of describing every detail of its construction, you mention solely that it had a cockpit, a wing, an engine, a serial number, and bullet holes in its body. This tells a reader all the important parts: it was piloted, built to fly, and most importantly, there's something even more advanced than this that shot it down.
The skill you need if you want to write short articles, is to condense things as much as possible without losing content. The distinct aesthetic of the SCP wiki can really help here. Summaries of anomalous or strange events, listed in an abrupt, matter-of-fact manner, can considerably improve a piece, and a single, bluntly-stated line can take a skip from 'ok' to 'great' with what it can imply. Consider SCP-3536. The addendum completely recontextualizes an otherwise bland SCP, and clinically states "yeah, people ate it. And it wasn't even the skip's influence, they just wanted to." And see how much shorter and cleaner that is than say, a note from a Researcher talking about it, or exposition-y journals from the perps.
I want to add as much information as possible to flesh out the idea
Now here's the crux of it all. You have to train yourself to think instead, "add only as much information as is necessary". This creates a 'tighter' skip; every word and every sentence will directly come into play over the course of the article, and nothing feels like padding or wasted space. Going back to the plane from earlier: do the exact schematics of the engine matter? How about the material of the seats? Or even the exact dimensions of the bullet holes? With proper implication, you can tell a whole lot using only a few words, and create an 'illusion' of a full story without writing it all out. If done well, and readers can visualize the whole story in the background, the SCP itself is less likely to feel incomplete.