Watch the Hammering Chords online guitar lesson by Vicki Genfan from 3D Acoustic Guitar
If you're having difficulty holding the 6th & 5th strings down with your 1st finger you can use your 1st and 2nd fingers. Practice building strength and dexterity in the 1st finger by moving up and down the neck while holding the 5th and 6th strings. Little by little it will become easier.( It can be tricky to get the finger to bend backwards.) If it really hurts, DON'T DO IT.
A lot of what makes this exercise work has to do with the left hand pinky hammering off and on the 4th string. Again, take the time to practice this slowly. It's a great way to build strength and dexterity in the pinky.
Be sure to take breaks and stretch/relax your hands whenever you need to!!
>> Techniques in this exercise:
1 - Thumb slaps
2 - Hammer Ons
3 - Single note tapping
4 - Group tapping
5 - Thumb brushing
That's a lot!!