As an ECB user here are the things I would do first. Drill at least a 1/2" hole in the bottom center of your charcoal pan. Purchase or adapt a grate allowing for a 1 inch or greater air space between the pan and the bottom of the grate. Also take your legs off and put them on the outside of the unit. Use some 3/8" x 8" bolts with double nuts and make legs for your charcoal pan. This will allow you to set the entire smoker around the fire pan. Not so much a problem when starting up but it allows you to lift the entire smoker, meat and all, off the fire pan when it comes time to change out the ash for fresh coals.
I set a my ashed over coals in an old juice can with the top and bottom cut out and then pack my charcoal around the can and drop a few wood chunks on top of the raw charcoal. I save one or two pieces for the hot coals. I then pull the can and place the last couple pieces of smoking wood. I then place and set my water pan, and grates, drop on the meat and then monitor my temperature.
A few words of advice, rub your butt and let it come to room temperature before putting it on the ECB, this will prevent a big temperature drop. Using boiling liquid for your water tray. This also prevents temperature loss and allows you to put the meat on immediately as the steam will allow the temperature to come to within range almost immediately. You will want a long funnel or a watering can to refill the pan as needed. I keep a pot simmering so I can add liquid as needed but usually only need to top off ever 3 hours or so. You probably want to double line your water pan with aluminium foil for easier clean up.
I would check temps every hour or so, you may need to knock down the ash using a pair of tongs. You should get 4-6 hours at temperature depending on outside air temp and wind. If you can set up a wind break, if it is windy you will have more consistent temps and be happier with the finished product. If you are only doing 1 butt count on 1.5 hours per pound. If you are doing more than one, or multiple meats count on 2 hours a pound for the largest piece. If you can get or you have a digital probe it helps alot with monitory meat temperature without having to lift the lid to stick in a probe. This always requires temperature recovering time resulting in longer cook times.
Also you might be able to add 15 coals every hour our so to help keep temps for a longer time period depending on how much ash accumulates below the grate.
I have made several mods to my ECB's and I compete with them so I know you can get a good product if you are patient and diligent. If you have any more questions let me know.