Here is the tutorial showing how you can do drafting of a selected part directly from assembly.
1. Open the assembly.
2. Start new drawing sheet.
3. Click on the ''Front View" option and through windows toolbar go to assembly again. (Keep the assembly open in other window.)
4. Select the part you want draft. It will get highlighted as shown.
5. Now select appropriate plane to orient the front view. It will show the whole assembly at the bottom right corner but still click on the plane.
6. After you click on the plane it will automatically open the sheet and you will see only single part in the drafting.