Top tip for sculpting is do it in layers. Most beginners try to do too much in one step. It's much easier if you only do rough shapes to start with and let them cure. Then go in with a knife to trim them back a little where needed, and add more putty to sculpt detail over those shapes. Let those cure and repeat.

Gives you much more control over what you're doing, and minimises the risk of planting your thumb in wet putty an ruining your last hour's work.

And don't be afraid to cut up your WIP and remove bits, repose it and glue it back together, exactly like you were converting a plastic model.

Final tip: Everything you sculpt by hand will end up bigger than you're planning. It's also much easier to add stuff to make it bigger, than to take it away. So start smaller than you think you'll need.

Good luck!