There are probably several ways of doing all of this. My approach:
If you have not done so already, create an account in your webspace with cPanel for domain.com. Create an HTML file (which you can call index.php) that is a frameset. Let the frameset comprise two horizontal frames, with the top frame being 0 pixels high. Link the bottom frame to http://guider.buildtolearn.net/subdir1/index.php
As for DNS, all you need to do it set the A records of domain.com to point to the IP address of your cPanel webspace.
2. I wish to create and link subdomain.domain.com to guider.buildtolearn.net/subdir2/index.php. I would like all the links to say subdomain.domain.com/index.php. How do I do that, step by step?
Create the subdomain in cPanel (log in at www.domain.com/cpanel)
. This will create a subdirectory subdomain inside you public_html directory. Repeat the above frameset trick with an index.php file inside that directory, this time pointing the bottom frame to http://guider.buildtolearn.net/subdir2/index.php
And again for the DNS, all you need to do it set the A records of subdomain.domain.com to point to the IP address of your cPanel webspace.
3. My current www.domain.com host also provides me with firstname.lastname@example.org. I understand question one involves changing ns1 and ns2. Question: will my email still arrive @domain.com after this change?
If domain.com has already been pointing to your cPanel webspace, then the above DNS steps were not necessary, and nothing will change as far as your email goes. However, if you needed to change to A records above, then you should do one of two things:
* To leave your email functioning on another existing server (not the cPanel one), change the MX record for domain.com to point to that server. Example:
domain.com, MX, 0, mail.domain.com
mail.domain.com, A, 220.127.116.11
(change 18.104.22.168 to the actual IP)
* To move your email to the server with your cPanel webspace, first use cPanel to create the addresses, and then update your MX record to point to that server's IP address.
I hope this helps