How to read RFC ?

Why ?

You want to know the more detail of technology or protocol standard.

Where ? 

The first thing you need to access the 

https://tools.ietf.org/rfc/index

How ?

You should know which one RFC is the latest RFC of your interesting topic.

1. Does it has been obsoleted by other RFC document ?
2. Does it has been updated by other RFC document ?
3. Does it has errata ?

For example:

You want to study about the topic 'Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile’.

Then you find the related RFC-3280 as https://tools.ietf.org/html/rfc3280

Firstly, you could find the document header of this RFC-3280 as the following:

[Docs] [txt|pdf] [draft-ietf-pkix-n...] [Diff1] [Diff2] [IPR] [Errata]

Obsoleted by: 5280 PROPOSED STANDARD

Updated by: 4325, 4630 Errata Exist

Network Working Group                                         R. Housley
Request for Comments: 3280                              RSA Laboratories
Obsoletes: 2459                                                  W. Polk
Category: Standards Track                                           NIST
                                                                 W. Ford
                                                                VeriSign
                                                                 D. Solo
                                                               Citigroup
                                                              April 2002

The first thing you have to check one thing “Is it the least RFC document ?” or “Is is obsoleted by other RFC document ?"

As you saw this RFC-3280 has been ‘Obsoleted by: 5280’. 

In addition, it has the errata updated by: RFC-4325 and RFC-4630.

And the RFC-2459 is obsoleted by this document RFC-3280.

Therefore, you should find the RFC-5280 first. 

Ok, now we go to https://tools.ietf.org/html/rfc5280 for the RFC-5280.

However, we find the document header of RFC-5280 as the following:

[Docs] [txt|pdf] [draft-ietf-pkix-r...] [Diff1] [Diff2] [Errata]

Updated by: 6818 PROPOSED STANDARD

Errata Exist

Network Working Group                                          D. Cooper
Request for Comments: 5280                                          NIST
Obsoletes: 3280, 4325, 4630                                 S. Santesson
Category: Standards Track                                      Microsoft
                                                              S. Farrell
                                                  Trinity College Dublin
                                                               S. Boeyen
                                                                 Entrust
                                                              R. Housley
                                                          Vigil Security
                                                                 W. Polk
                                                                    NIST
                                                                May 2008

Therefore, you should read the RFC-5280 for the topic 'Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile’.

In addition, you should also check the RFC-6818 for the updated, and also check the ‘Errata’ part for this RFC-5280