The two biggest mistakes folks make with brisket is relying on time and to an extent internal temp(IT) of the brisket.
Time is almost irrelevant and IT is nothing more than a guide to let you know when to start checking for doneness and that is when the meat is probe tender.Having a good thermometer is key to let you know when certain things are happening in the cook,a multi-probe is beneficial as it allows you to monitor both the meat and the smoker temp.For your first point time really doesn't matter which brings us right to the next point.You wrap the brisket when it reaches the "stall" ,without getting to scientific here it's the point in the process where the IT stops to rise.Some just let it ride others wrap to push the process,if you're gonna wrap do it at the stall.The stall can start after just a few hours up to many hours later,generally it tends to happen in 160°-170° range but each piece of meat is different and it can certainly happen outside that range.Adding a little liquid to the wrap can be done if you so desire but I personally don't.
When to start checking for doneness (probe tender)?Again the IT is a guide for this,I generally will start probing for this at 195° but again each cut of meat is different and this very could be as high as 210°+ . Probe tender is when the probe which can be a skewer,a handheld temp probe or even a toothpick goes through the meat like warm butter.Probe tender is when to pull your meat.
Resting is pretty simple.I let it sit on the counter until the temp drops to about 170° or so and then I wrap in foil,then a few towels and then into a cooler for a very minimum of an hour but you can hold up to 4 hours or more if needed.
Water pan and spritzing,for a lot of folks that depends on your smoker type and in many cases just personal preference.When I use my electric smoker I don't wrap and I personally have never spritzed my briskets.
Fat side up/down?That's a debate that's been going on for a long time,in the scheme of things I don't think it really matters.
As far as your results being tough,as stated above it's underdone.
Some advice,225° isn't a magic number,I do my briskets at 250° - 275° which shaves a few hours off the overall cook time.Get yourself a good quality multi-probe thermometer with hi/lo temp alarms,it's the best tool you can have in your arsenal especially when doing cuts of meat where the IT and pit temp are critical.
