So the state(s) process of secession was set out (documented) ahead of time somewhere, correct? Otherwise how would the People know it was being done properly?
Did every state have its own procedure for doing this, or did they all follow the same set of rules?
Was any provison made for what would happen if they didn't follow the correct procedure?
After all they wouldn't want to make such changes for light or transient causes, right?
No. Each state followed it's own internal requirements for ratification and secession, based upon centuries of English precedent/colonial actions/state ratifications.
Did every state have its own procedure for doing this, or did they all follow the same set of rules?
See above. Some states voted for representatives in varying manners (by district, counties etc), with varying numbers necessary etc), there was no one perfect method.
Was any provison made for what would happen if they didn't follow the correct procedure?
No - there was no incorrect procedure - even if there were - they states, as parties to the compact, could have protested any dubious ratifications. The only stipulation was that ratification occurred in a convention of people from each of the several states separately. ("The Ratification of the Conventions of nine States ..."). It's not anything new to the states, as each had ratified it's own state Constitution previously.
After all they wouldn't want to make such changes for light or transient causes, right?
Who determines what is light or transient? Ask Rhode Island & Providence Plantations, which didn't even send delegates to the convention. It only took them 14 attempts before the finally ratified, and then only because they & North Carolina suffered threatened economic sanctions for failure to ratify. Either way, there is no requirement proscribed in the Constitution.