Unfortunately I can't really help there, I'm a "learn by doing" type of person who just jumps in the deep end and hopes he figures out how to swim.
Most of my learning was just "capture the problem happening, capture what happens when it works right if possible, open up the relevant RFCs, then try to understand what's different and why.
I work in the VoIP industry so I'm dealing with a lot of NAT problems (insert rant here about lazy ISPs that still haven't enabled IPv6 on their networks) and my main protocol (SIP) is heavily inspired by HTTP and as a result is more or less human readable plaintext, so it was a relatively easy learning curve to just have Wireshark open on one side of the screen and the relevant RFCs on the other side.
All I can really say is have a problem you want to solve and start from there.
Most of my learning was just "capture the problem happening, capture what happens when it works right if possible, open up the relevant RFCs, then try to understand what's different and why.
I work in the VoIP industry so I'm dealing with a lot of NAT problems (insert rant here about lazy ISPs that still haven't enabled IPv6 on their networks) and my main protocol (SIP) is heavily inspired by HTTP and as a result is more or less human readable plaintext, so it was a relatively easy learning curve to just have Wireshark open on one side of the screen and the relevant RFCs on the other side.
All I can really say is have a problem you want to solve and start from there.