aseerel4c26
Welcome to OSM! You know how the building looks like and/or can visit it again to have a look at its proportions? Fine! :-)

How-to with the default editor: log in, click "edit", draw an area where the building is and then select building on the left side. Then "save" and please enter a comment describing what you did and what your source for this is. An advanced step (maybe later) would be to add a so-called "source tag"  to the building itself to tell fellow mappers that this building is just estimated.

Just try it! Tell us here if you succeeded or need further help.