I'll answer the second question first.  How do you know when to block?  The quick answer is "Any time you hear sounds you don't want to hear."  The purpose of blocking, of course, is to eliminate sound.   But just when to use it is almost a matter of taste.  It is hard to give a hard and fast rule. The amount of time you allow a note to ring before you begin another is tough to teach.  It's more of a feel and ear thing.

You have to develop an ear for blocking.  You will pick that up as you work more with the tablature or listen to other players.  You will hear how long notes are before you mute them and go on to the next one.

Now as far as blocking or mute techniques, here are the ones I use.

• I block with my picking hand (palm blocking)
• I block with my picks (pick blocking)
• I will mute a string while other strings are ringing out, with a single finger
• I will also mute notes with my thumb of my bar hand.

So there you have four different blocking techniques that you can use and if you see me play, you will see I use every one of them every night. Now, I did start with palm blocking, but as I learned, I decided it was important to learn as many of the techniques as possible. I believe there is something to be offered from all different techniques, but it can be mind-boggling to learn them all at the same time.

I teach my students two different blocking techniques (palm and pick blocking), right from the start. Many will tend to favor or be able to produce one of the techniques better than the other, but they are capable of doing them both if needed.  So I would say practice all the techniques that DeWitt has in his book as he is a brilliant teacher, and when the time comes to play something perfectly, you will use the one that is most comfortable for you for that particular passage.