There are two types of Slants, forward slant and reverse slant. There is a distinct technique to practice each type. Lets look at a forward slant first using this tablature.

E__12~~13}    ______________
C________}    __7~~9~~11~~12
A__12~~~~} OR ______________
G________}    ______________
E________}    __7~~8~~10~~12
C________}    ______________

I like to place my fore finger tip on top of the bar as a pivot point. The second finger is behind the bar on strings and the thumb is towards the bottom of bar on other side of second finger.  Use your thumb to push the bottom side of the bar back as, at the same time, you push the nose of the bar forward with second finger.

Now for the reverse slant, use this tab to practice.

E_10~~7~~8}    ___________
C_________}    _17~~11~~12
A_10~~8~~7} OR ___________
G_________}    ___________
E_________}    _17~~13~~12
C_________}    ___________

Again with fore finger tip on top of bar and the same position for the second finger and thumb

1. Play the notes. 
   
2. Take the thumb tip to the bottom of the bar first and as you slide down to the slant, push the bottom of the bar forward with the thumb
   
3. Now the second finger tip goes to the nose of the bar and fore finger stays on top of the bar.
   
4. To finish to either a straight bar or a forward slant, the thumb pushes the bottom of the bar back into next position.

Practice these exercises until the fingers and bar work together in one motion and always remember that the thumb initiates most of the movement of the slant.