If you're going to a convention yourself, you'll have to take the slab to the CGC booth and have the slab cracked and the book removed in front of the CGC witness. Then the signing itself is rather academic, the CGC witness accompanies you to obtain your sigs, you and the witness return to the CGC booth and have the books submitted.
If you're using a facilitator, they will crack the book from the slab for you and will obtain the signatures before submitting to CGC.
In both cases, if needs to be emphasized that the book is to remain in the slab and can only be cracked under the watchful eye of a CGC witness/authorized facilitator. DO NOT crack the slab yourself beforehand.
As for the pressing, if you elect to go through CCS, you just mark it on the Submission Form and it will flow through CCS before going to CGC. If you want to go with a 3rd party, I would recommend working with your facilitator to sort out the details.