Foreword; Beta testing is an integral part of any mods progress, a mod team, no matter how big, can rarely spare the time to properly test every aspect of a mod. Hiring testers who can be trusted is a big step. For most teams this will be the first time they have passed their labors onto a complete stranger and they will hope, fingers crossed, they find people who will take the role seriously.
The main factor is always time. The team will often be waiting for your feedback before they can progress so taking a two week holiday the day you DL a mod isn't going to go down to well. Bear in mind you may be new to the team but the modders themselves will have already put hundreds of hours into getting the mod to the beta stage. ect ect..
1: Play the faction you are asked to play rather than the one you want play. The Mod team have taken you on because they need feedback on a specific faction and while playing as the big boys may be tempting balancing requires all factions are given equal attention.2: Do not tweak stats on the version your reporting on. Ok so maybe your Spartan generals are not the 'uber' killing machines you feel they should be. Tweaking them to your tastes won't give the team a true reflection of how they will preform in the final release.3: Don't give unauthorized people access to the beta. You may feel an overwhelming desire to share you beta with other members, but imagine if the beta falls into the hands of someone else who has an overwhelming desire to share, before you know it the beta is sitting on a thousand desk tops.4: Being honest; the team hope they have made a well balanced mod which looks great and rocks your world, they are in fact quietly confident this is the case. If not say so and why and, if you can, ways to make it better. Endless posts of ''oh my god you guys rock, I love you, this is the best thing since garlic bread was invented,'' though it may be a nice ego boost to some Isn't really that helpful.5: 50 turns syndrome; (unless they state otherwise) Chances are the mod team has tested the first 50 turns of the faction you've been asked to play a hundred times. They would probably like you to skip the 20 page summery of how you got out of the financial hole of the start position and, how you defeated ten local rebel cities.6: Dropping out; Sometimes going A.W.O.L. just can't be helped, please try and drop a PM to the team if you think you have to take off, it's not uncommon for a team to be waiting for a report that is never going to come which can be very frustrating.7: CTD's and reporting them. You almost certainly will come across a few CTD's when testing. Assuming the team have done some testing, the campaign loads up and all seems well. Common issues arise once characters start moving around and interacting with other elements on the map, ie; settlements or other characters. There are a few things you need to do here which will help the team bug hunt.
A; Save your game after each turn.
B; It's a good idea to keep a list of your armies units, especially those moving around the map.
C; If attacked or attacking it's a good idea to make a list of the opposing army.8; Also, try to always provide screen shots. Saying "I saw snow in the desert" is a first step, providing the screen shot may provide visual information a text explanation does not.9; save game each year / turn - with different descriptive names for each like "sev-hh-y980" = to mean Severin campaign, on hard, hard at year 980 etc.... so you don't overwrite old ones and you can find relevant ones in-case someone asks you to look back... (this is general good practice, but not something I often managed to persuade most of testers to do, Halie was excellent beta tester and exceptionally funny) ..
Ending up with one final save / autosave which proves that it won't load / advance to next year isn't always exceptionally useful, you may need to go back to look at prior saves if available....
And finally, try everything. Don't just do the basic things everyone will do. Even if it's against your playing style. Things like trying to move anywhere around the map can help find mapping problems like blocked pass, rivers unable to be cross...
See posts below for contributors to the OP' and other tips for beta testers.