The quick answer is, it is pretty simple! Just apply a 301 redirect to the URL that is leading to a 404 error page.
But first, let me explain a few things about 404 as some of the terms used here might really confuse some people (please watch the first video below).
There are a few videos that explain various aspects of 404. You don’t need to watch them all to find what you are looking for – you can skip straight to the videos that talk about how to fix the problem – but understanding what is 404 and why 404 errors could be bad for SEO is very important as well.
What is 404? 404 errors? error 404 page not found?
Generally speaking, 404 is a server status code that indicates the page requested cannot be found. In many cases a 404 error page is shown when a user gets to that page.
It is generally referred to as “404 errors” among search engine marketers.
How are 404 page not found errors generated?
If you type any random URL (for example http://digitalreadymarketing.com/randomurlthatdoesnotmeananything) you will be brought to a 404 page. In other words, if the URL does not exist, a 404 error is generated.
It is very common to have 404 page not found errors on a site, it happens ALL the time. Especially when you edit URL of a page or delete a page. In WordPress, unless you change the URL manually, updating the title of a page itself will not alter the URL. However, a lot of time people want to match the URL with the new title and this is when a 301 redirect should be implemented to tell search engines and users that the page has moved to a new location (more about that later).
Also, increase in 404 page errors happens when migrating a site. The best practice is to map out the new and old URLs so 301 redirects can be implemented accordingly.
Why 404 page not found errors are bad?
If you look into Google Webmaster Tools > Crawl > Crawl Errors, you will see Google actually take notes about the 404 errors on your site.
There are at least two ways Google (or search engines in general) find these URLs.
1) When search engines come across to internalins on your site.
2) When search engines reach to your site from links on other sites (a.k.a backlinks).
As you know backlinks is a ranking factor so the more backlinks you have (ideally quality back links), the more likely your site will rank higher. So imagine what would happen when one of your pages with many backlinks disappear…. you guessed it right, those backlinks are now connected to the 404 error pages and getting wasted.
This is why it is important to 301 redirects to redirect search engines and users to the new page (note: use 301 redirects and 302 or 303).
How to find 404 errors?
There are various tools you can use. A very good starting point is to use Google Webmaster Tools. It’s free and it tells what Google is seeing about your site.
You can also use crawler such as Xenu or Screaming Frog.
If you want to stick with WordPress plugins, SEO Ultimate has a 404 monitoring module.
How to fix 404 page not found?
You need to identify them first, then find a closely related page (or as closely as you can) and apply a 301 redirect to the old URL so search engines and users are redirected to the new URL.
Redirection is a WordPress plugin that can help you achieve this at page by page level. It is very simple to use (watch the video).
It is not recommended to redirect all the URLs back to the home page – in case this is what you are thinking. It will create a soft 404 problem which is also reported in Google Webmaster Tools. If Google takes time to tell your site has soft 404 issues, you can imagine Google really doesn’t want your site to have that issue!
What is soft 404?
Soft 404 happens when the page appears to be an error page yet the server status code is showing 200. This sends a mixed signal to Google and since Google does not like wasting its resource, it wants webmasters to fix these issues.
When you redirect from a page to completely non-relevant page, Google thinks the new page is so different and non-relevant to the original that it flags the page as soft 404.
The way to fix soft 404 is the same as other 404 error pages. 301 redirect to a relevant page.
This is why redirecting everything to the home page is not a good idea because it will cause a massive soft 404 issue. When migrating a site, sometimes it is inevitable since some webmasters want to use “catch all” method to redirect everything to a certain page. Even though it is a good short term solution but in reality it only converts the “404 not found” issue to the “soft 404″issue.