Firstly don't panic. Brisket is just another meat that needs to be cooked slowly. Most on here have their own methods for "the perfect" brisket but providing you follow a few basics then you will be fine.
The brisket will take time and likes a constant temperature. All of this time does not need to be in the smoker though and it likes a good long rest towards the end. You are trying to get all of that tough collagen that is in the meat to break down - which takes time.
What is your brisket like? The thicker the better here in the UK (they can often be very thin). If it has a lot of fat on the surface then trim a lot of it off. Just how much is up to you. Some like to keep a good layer of fat on the top however I like to trim mine almost clean. I also do not like the flavour of the fat under the flap and so trim most of that out too. The more fat you leave on the harder it is to dry out however as I foil mine part way through drying out isn't a problem.
Once trimmed you can either apply your rub directly or first use an acidic marinade to open up the pores in the meat surface. Applying the rub of choice directly to the surface works fine. I use an acidic marinade (which is essentially a mix of 25% cider vinegar and 75% water in which I mix a couple of tablespoons of the rub) and this is rubbed over the surface of the meat and left for 10 minutes before then applying the main rub. The marinade step is optional.
Place in the centre of the smoker set at 225 F with a drip tray underneath and leave it smoking for 3 hours. At this point the smoke will have already done its job as a flavouring and you can foil. If you decide not to foil then you should baste it or spray it with the rendered fat or with apple juice every hour or so to help keep it moist.
After a further 3 hours (6 hours in total) remove it from the smoker, double wrap it in foil, wrap it in a big towel or blanket and then put it in an insulated box (a camping cool box is great.
If you decide not to foil then leave it for the 6 hours and then foil it, wrap in the towel and place in the cool box.
Leave it to "rest" for at least a couple of hours - more if possible. It will continue to gently "cook" under its own heat, breaking down those fibres and releasing all of that flavour.
When you are ready to serve, carefully open the foil and pour out the collected juices. These make a great sauce or gravy and can even be frozen for later use in other meals.
If you want to firm up the outside before serving (which I don't) you can simple place the whole brisket back into a hot smoker or oven for 10-15 minutes.
Keep it simple, keep a constant temperature, and keep patient and you will have a brisket to be proud of.