I am taking it as read you can make the F chord shape and you can place all fingers down on the strings at the same time with ease.
Ok now try this little exercise, ONE FINGER AT A TIME starting with your ring finger LH place the finger on the forth string 3rd fret, next the middle finger on the 3rd string 2nd fret, next the index finger on the 2nd string 1st fret lastly the pinkie on the 1st string 4th fret. Now check to make sure all your fingers are down on the fret board in the F chord position. Adjust if necessary.
Repeat this exercise 50 times.
Now we are going to add the right hand, Start the exercise again but this time pick the forth string with the thumb after you put down the ring finger.
Put down your middle finger 3rd string 2nd fret
Next put down your LH index finger on the 2nd string and pick the 2nd string with your RH index finger
Then put down your pinkie on the 1st string and pick the string with your middle finger RH.
With all your fingers down in the F Chord pick the 5th string with your thumb then Middle, Index Thumb to complete the roll.
Now practice this exercise slowly 100 times or until you have it committed to memory.
This exercise may seem a little tedious at first but you should realise it’is preparing you to be able play more accurately and faster. It’s not an exercise in building speed per say but in time your speed will increase naturally.
In his reply Dave suggests using a Push Off rather than a Pull Off. I would agree with his advise whilst your learning this lick combination and go with the Push Off. Personally I use both and I think Ben does too. But I started out with the Push Off. The reason this is better is because your moving toward the forth string with that ring finger so it’s easier to place.
I have learned (over many years) to Pull Off with the Ring Finger (because I get a better snap) but it means that I have to move much more quickly to land my ring finger on that forth string in time to start the roll.
When your learning to make chord shapes your told to practice putting all fingers down on the strings together. This is a good way to learn your full chord shapes. But in point of fact when you play the banjo up to speed you rarely put down a all your fingers at the same time. More often you will make a chord in the way I have described in the exercise above one finger at a time (on auto pilot) and In time you will learn how to make partial chords using less fingers which means you can play faster still.
I hope this helps and goes some way to reassure you that we have all been through this frustrating learning process. We are all itching to learn to play all these tunes up to speed and error free. Relax, take a deep breath and start again. You need to have patience and perseverance if your going to succeed with the banjo.