Ajgall,
You didn't mention what type of lock you are planning on assembling or the source of the castings. As a result you've received some good but pretty general advice. You also don't give any details on the type of tools and equipment that you have available.
There are at least two ways of learning a task or skill. One is trial and error or the "School of Hard Knocks". The other is studying published "how-to's" to learn from them.
A couple of resources in the "how-to" category is John Bivins' article titled "Precision Assembly of Lock Kits" that was originally published in
Rifle - The Magazine for Shooters, Jan-Feb 1977 issue #49. Sometimes you can find these old magazines on Ebay. The article was republished in
Gunsmithing Tips & Projects. The
second edition of this book is available from Track of the Wolf.
An online source that can be helpful if you have access to a lathe and mill or drill press can be found on the
ctmuzzleloaders.com website under
Building a Manton V-Pan Lock - Part 1 - 3. This example uses a set of castings from The Rifle Shoppe.
These resources show different solutions to a common problem--how to hold the odd shaped parts to machine, file, drill, tap, turn, etc. Bivins shows how to make a "holding jig" that holds the lock plate for drilling, tapping, and reaming holes. He also uses clamps to hold parts such as detachable pans and bridles to the lock plate while it is held in the jig. Bivins jig is designed to be used with a drill press. The guy assembling the Manton lock uses high-temperature shellac to hold parts to brass round stock and brass plate for various operations.
The image below is from Bivins' article showing the tools commonly used. The caption for the picture is below the image.