At the time I put on the arm size I was actually training full blown DC, which isn't what I do now, although I still stay true to the DC principles when I do train my arms and these days only directly train them once per week.
I used the intermediate DC split, so I had two different days and trained 4 times per week. Mon & Tues, Thurs and Fri on. Wed and weekend off.
I would train biceps twice per week and triceps twice per week. I would have 3 different exercises that I would rotate through, performing a different variation on each workout. So just for arms (this doesn't include the full split) it would look like:
Monday: seated incline DB curls
Tuesday: close grip bench press from pins in rack
Thurs: ez curl bar preacher curls
Fri: barbell skull crushers
Monday: Pinwheel curls
Tues: ez curl bar incline overhead tricep extensions
Thurs: seated incline DB curls
Fri: close grip bench press from pins in rack
etc, continue rotating between exercises. Choose whichever you prefer.
The magic really isn't in the exercise selection or the frequency although hitting a muscle twice per week seems to be ideal for hypertrophy for most.
The magic is the way that you execute each set. First ramp up your weight, for whatever lift you're going to use. So if Im doing incline db curls, I'd start with 25 lbs and do 5 reps. Then 30 x 5 and then 35 x 5 etc.
You ramp up to your working weight where you can do 15-20 reps, let's say it's 40 lbs DB's.
Example: You hit 17 reps and couldn't squeeze out another rep if I held a gun to your head.
Drop the weights and rest for 15 seconds (rest pause sets). Pick up the weights and go again. This time you only hit 8 reps. Drop the weight and rest again for 15 seconds. Go one more time, this time you get around 4 reps.
Now you're done and you stretch that muscle intensely right away for 1-2 minutes. Some of the stretches work best under weight, like triceps, I'll take a heavy dumbbell and stretch my tricep with it behind my head.
Keep logs and try to add a rep every workout. After enough added reps, increase the weight and obviously expect your reps to decrease.
It's ball busting but it's effective.