From your assembly, file>save as>part.
Details have been discussed many times on the forum, a quick search should give you plenty of info.
I knew it was simple but I was looking for a complex answer.
if you are at all concerned about what the recipient can learn from the step file, I recommend opening it up before you send it and see what you can see using sectioning and such. you mafind you will want to do a little more to hide your IP - we do .
Retrieving data ...