When creating custom item editor in Sitecore(Editor tab), only required field is the Url, which will point to the .aspx page. Where this .aspx page should be added and what URL should I type. I've created a web application in Visual Studio and an aspx page with the desired functionality in it. This web application isn't related with my sitecore project. How to specify the URL to this page. Or should I add this page somewhere to sitecore project? Thanks in advance.
Go to core
database, then in content editor go to /sitecore/content/Applications/Content Editor/Editors
item. Create new folder item here if you like and inside create new item with this template: /sitecore/templates/Sitecore Client/Content editor/Editor
(you can copy any existing editor item into your folder). You can set header, icon and Url (any url, absolute e.g. http://stackoverflow.com or relative e.g. /sitecore/shell/Applications/Media/Media Folder.aspx
).
Now go to master
database and set this editor in Editors
field (inside template or specific content item)
I think you should move your Page.aspx
inside sitecore project (in layout folder or custom one)
then url should be /layouts/Page.aspx