You need to register as a contractor within the Construction Indusrty Scheme (CIS). You can find out how to do that starting here and following the relevant links. The other sections tell you how to make deductions and account for them to to HMRC. HMRC will set you up with a CIS payment scheme or convert your existing PAYE scheme to a PAYE/CIS scheme if you have one. Sectrion 5 here deals with how to make deductions from your sub-contractors pay.
I hope this helps but let me know if you have any further questions.
Take a look at section 5 here. The link was in my previous post.
You deduct 20% tax from your subbies if they are CIS registered and 30% tax if they are not CIS registered.
Everything you need to know to comply with the CIS rules is in here. Have you clicked on the blue links I gave you previously? Here is the full link on deductions from subbies:
As your subbies are CIS registered you deduct 20% tax from them. I cannot be clearer than that.