So, even just after today's teachings, if we become members of DC, we can start practicing the Semdzins, Lojongs and Rushens to take away all doubts?
You don't have to become a member of the DC to do that. But in general, yes, if you are interested you can buy books, and so on after becoming members of DC. Song of the Vajra, incidentally, is a perfect semzin. You should purchase precious vase, also there are short booklets that explain these things very well. Then you try to meet people who are nearby who are in DC and ask for their help in showing you how to do things, or you buy DVD's etc, for those practices in which you are interested.
Is that what you would recommend for someone who is still unsure about Rigpa?
Practice Guru yoga.
Do the booklets provide accurate enough descriptions of HOW to do them?
For example, with the Guru Yoga of the white A, is there a special place on visualises the A or not at all? I didn't completely understand that.
Center of your body.