You go to the country (in the country list) or search it at the top, then in the white spaces you put the number of coins you have of that type, taking in count his state.
In other words: go to the coin page (click on the denomination or image if starting from country page or search results page, on description line if starting from top search bar), then look for the correct date and variety (there should be descriptions on the coin page), and write a 1 (or however many you have, if you have more than one of the same coin) in the box corresponding to your estimated grade for the coin (or, if your estimate is below VG, in the VG box, because Numista doesn't allow lower grades so far).
In some (hopefully rare) cases, the variety distinction is not explained well enough on the coin page, and/or your coin is in too low a grade to tell which variety it is. In this case, it is apparently allowed (as long as you're certain of the type itself) to add your coin under any of the varieties that it could be (if applicable, I prefer using the one with mintage information, the highest mintage, and/or not labelled "rare"), and in the "personal comment" field (indicated by a three-dots icon immediately right of the boxes for the respective variety) add a comment explaining that you're not sure of the variety (and, preferably, why).
If your coin is not in the catalogue at all (or is as a type, but your date is missing), you can try to add it. But that's another problem entirely (I'm still not sure how to do that).
There is a simplified way to do it without clicking on each result if you are purely looking to add coins to your collection.
When you search and get the results, look for the "Fastinput" option on the right and set it to "Show", this will enable the options under each result to show the check boxes to add your collection. Simply put the number of coin/s you have for one year/type in the relevant grade boxes (no need to click enter, they are auto added as soon as you type in a number. (it may just be me, but you may have to click on each box twice and then you can enter a number)
Restoration addict : Verdigris Removal : Zinc White spot removal : Iron Rust Removal : Silver brooch/necklace mount Removal