How are the fixtures worked out?
Why don't all the teams in each conference play each other throughout the season?
For example Chicago Bears (NFC North) play the other teams in the division Home and Away and then play cross-conference teams.

Indianapolis AFC South
Carolina NFC South
Tampa Bay NFC South
Philidelphia NFC East
Atlanta NFC South
Tennesee AFC South
Seattle NFC West
Jacksonville AFC South
New Orleans NFC South
Houston AFC South

Can anyone explain the fixtures??

There is a rather complex formula that the league schedule makers go through.

Every team plays a home and away within their division. That takes up 6 games. They also play games against a division within their conference and outside their conference. The remaining two games have to do with their previous year's record.

Yhe other division games work on a rotating basis. This article may be of interest:

NFL Schedule, Rivalries and Potential TV Ratings Optimized by Packaged Software - CIO.com - Business Technology Leadership