I've done many reviews of guides in the past, and the most common suggestion that I have for guide-writers is to establish your credibility as a guide-writer. If you are writing a guide about a champion, then you need to prove to your readers that you know what you're talking about.

For people who are higher up in the league system, establishing your credibility is pretty easy. Its just as easy as verifying your ranking for the most part. Sure the additional statistics that you can pull out to establish your credibility further is great, but proving to people that you're part of the top percentage of LoL players should be fine.

Now for people are not as high up in the league system, you will have a much harder time establishing your credibility as a guide-writer. The first thing that you need to do is explain why you should be listened to over somebody who has a significantly higher rating than you. There is a lot of ways you can support your credibility without having a high ranking, here are a couple:
  • Match History. This is relatively common in guides nowadays, and provides a decent level of support. I highly recommend that if you decide to support your credibility with match history that you request for guide-users to submit their match history. If you can prove that people who have used the guide have had success, then that gives people more motivation to try out your guide. As Nighthawk pointed out, try to include the summoner names of other players in your normal match-history in an attempt to determine normal elo.
  • Ranked Statistics. You can be whatever ranking you want, but champion statistics mean a lot. If you have strong ranked statistics with the champion you are writing about, then you should share it. Strong ranked statistics consist of a combination of a positive win/loss ratio as well as a large number of games.
  • Professional Talk. I have not seen this very often in guides, but it is a somewhat feasible way to show your credibility. I wrote an Anivia guide a while back, and the primary source of credibility that I had as a guide-writer back then was observing Froggen's Anivia play on stream and in tournaments. If you mention a professional player who inspired your build/guide, then that usually correlates to a build/guide that is more accepted by the public. I bet if you mentioned Guinsoo's Rageblade on Aatrox is a viable item that some people will immediately go against you on that. However, if you mention that Guinsoo's Rageblade on Aatrox is a viable item, and has been purchased by GG Darien in competitive play while playing Aatrox, then people will probably not question your decision. As C4 Lasty pointed out, it is also very important that you understand the decisions that a pro-player makes prior to referring to them in you're guide.

Conveying these statistics to your guide-readers is also very important as pointed out by Veng Lmfao. You need to make sure that you don't come off as overconfident or stubborn when presenting your credibility, but rather you should try to come across as knowledgeable and welcoming. If you come off stubborn when writing your guide, most people (including me) will just not review your guide in the first place. Showing that you're stubborn and are not open to other ideas means that the likelihood of you listening to suggestions from others is slim to none.

All in all, every guide writer should make an attempt to prove to their readers why they should follow his/her guide instead of others.

I apologize for such a long blog post with no TL;DR :-/