Creating a friendly 404 error Page using web.config and umbracoSettings.config (Umbraco 7.4,Umbraco 7.6)
Firstly, In the web.config file, change the existingResponse="Auto" to existingResponse="PassThrough" on the 'httpErrors' line:
Secondly, you'll want to create your 404 page. Once the 404 page has been created, get the node ID of the page (you can find this ID in the properties tab when editing your page).
Once you have the node ID of your page, in the umbracoSettings.config file you'll want to add the following (be sure to add the node ID within the :