We are concerned here, as has been mentioned, with our relationship to our instrument. The plectrum introduces a new element into this equation: a small, flat triangular piece of plastic. To use it well it has to become a part of your hand.
Up until now you've been concentrating your focused awareness skills on various parts of your body. With these simple exercises you'll learn to extend that focus to the part of the plectrum that strikes the string. Make sure, as always, that as soon as you notice any muscular tension you slow down and reassess your approach.
Because we're introducing this third element into our playing it's worth taking some time to work it into the mix. Firstly be aware of how you hold your pick. This is one way that works well, curling your third, fourth and fifth fingers into a light fist is also common a good way to go. Most competent players will have developed a technique that suits them and presents little problems, if you're not sure go to the forums, or get in touch for a consultation.
how to hold a flatpick
Now that you've got that sorted take your guitar and play consecutive down and upstrokes, on the same string at a slow even tempo. The great advantage of using a flatpick is the volume and the attack that you get on a note through the sharp impact on the string. This impact is not only audible, it's tangible as well, you can feel the plectrum's response.
Now extend your focus to the plectrum itself, particularly that part of the plectrum that impacts the string. Don't try and do anything else, simply keep playing that one note, ensure that you're shoulders and arm are relaxed and keep playing at a slow and even tempo. Keep focused on the edge of the plectrum and treat the exercise like you're playing a drone.
You should warm up with this exercise every time you play. It may seem rudimentary because it is. It's fundamental if you're flatpicker and the more you are in tune with this essential movement the better you'll be able to approach the rest of the exercises and eventually the way you play your guitar.
All of these exercises can be played using the above approach. They progress from the simple one note, one string to more complicated cross-picking patterns that will improve your proprioception. They all use consecutive eighth notes so you need to alternate between down and upstrokes in every exercise, I've only notated the first 2 bars of the first exercise, keep the same picking pattern for all of them.

Scales and arpeggios are also great for developing your picking technique, and more useful musically, it will take some time to extend this sort of approach to picking to more complicated phrases so just take your time.
The next few exercises are in 9/8. They are useful because they are (probably) foreign and if you can get them right you'll have added considerably to your neuromuscular memory banks. What makes them challenging is that you have to start each bar on a downbeat having just finished the previous bar on a downbeat.

Your right hand more than your left will dictate timing and the following exercises will help you develop your inner sense of timing and importantly extend your focused awareness skills to new aspects of your playing.
Play these simple rhythms in time with the backing track. After a few bars the backing track will take a one bar rest. You need to keep playing and make sure that you come in spot on the first beat of the bar after rest precisely in time. The first one has a number of one bar rests, the next one starts with one bar rests and then has some two bar rests, the third one extends to three bar rests.