Everyone knows if you want big arms you need to train your triceps hard.
The tricep is 66% of your arm. There are many routines to stimulate tricep growth. The most effective routine in my opinion is the one below.
Skull Crusher- 3 sets of 6-10 reps
Close Grip Bench Press- 2 sets of 6-10 reps
Cable Push Downs- 1 set of 3-5 reps
This is a simple routine for triceps but it is very effective. The skull crusher is the king of tricep exercises. Nothing will give you a workout like 3 sets of skull crushers. The second most effective tricep exercise is the close grip bench press.
This will really work the outer part of your triceps. I only recommend two sets of this simply because you will already be very worn out from the skull crushers. Doing more does not equal more growth. The last exercise I highly recommend is the cable push downs. This exercise will work the long part of your triceps.
Since you have already done 3 sets of skull crushers and 2 sets of close grip bench press I only recommend doing 1 set of cable push downs. The other two exercises are more important in my opinion hence they deserve to be down for more sets. That does not mean you should skip cable push downs and do more of the other exercises.
If you can do this tricep routine for a couple of months, you will see nice growth in your triceps. Just keep it up consistently.