Screenshots can be taken in many ways. I think sothis still uses virtualdub(mod?), whereas I used to do it with Zoom Player. I even had a lot of custom keyboard shortcuts added (for framestepping, etc) to make it simple... Well, I still use them but I take the actual screenshots using the method explained here
http://www.cccp-project.net/wiki/index.php?title=FAQ#How_do_I_capture_screenshots.2Fgrab_frames.3F except for anamorphic video I would use VLC player's snapshot feature because I can't be bothered to manually fix the aspect ratio. Well, not like that's something you'd need to deal with that often if you're dealing with fansubs... You can use whatever you find the easiest.
As for the content of the images: no subtitles showing, have a scenery shot or few, have a few candidates for the main image for the series (named, for example, main1, main2, etc), take enough pictures so sothis has enough to choose from. Note that the main images in anirec are square shaped (and the rest are 4:3): scenes that look good in widescreen (ie, where characters are at the edges of the image) might not work at all in anirec because they need to be cropped. The main image should ideally have something happening in it that describes the series well (reference picture:
https://www.anime-planet.com/anirec/337.html), not just simply as many of the main characters you can cram into one picture.
There are some requirements regarding the content and the quality of the synopsis part as well. See the previous link for some hideous typos and grammar that won't fly anymore (sothis: wasn't there any talk of fixing this one when I took the new screenshots?). You might be asked for full or partial rewrites, and/or your entry might be radically edited. Just so it won't come as a shock later :)
As an amusing anecdote, the entry for Iriya no Sora, UFO no Natsu took three complete rewrites by yours truly, and was THEN edited by sothis before making it into the anirec :/
As for the style and content, read existing entries. I often think of them as blurbs (the short bits of text at the backs of books) and pretend I'm writing one. The target length of the entries used to be quite strict (what Frost said), but apparently longer entries are possible under the new system. Sothis can tell more about that...
I know that was a lot of "rules" to follow. Don't fret about them too much, though: sothis (and occasionally others) can help make up for whatever you missed. I merely listed it all (or what I remember of it) in case you want to be
really, really helpful :)