Hi, it is really very simple - the existing external drive will become your internal C: drive when cloned.
With the drive connected simply open Acronis and choose the Clone option - then just follow on-screen prompts without making any changes, or in other words, just use all the programs Defaults. Don't set a password or anything else, it really is just a matter of choosing Clone then let Acronis get on with it.
Depending on the amount of data and programs etc on C: it will probably take about half an hour or so to complete. The cloned drive will then be an exact copy of your present C: drive in every detail, except of course the Drives' Capacity - cloning is usually done to upgrade to a larger capacity drive.
You then just physically swap the drives over, at your own convenience.