Hi, A couple of things to note...

You are very vague and you seem to be asking how to do something with little or no study at all.  It is very difficult to jump right in and tell you the entire realm of dimensioning.  If you have not looked at some online video content or a light reference guide yet then I recommend you start there.

You are running on a Mac so good luck finding a lot of content for lessons on the Mac version.  There's stuff out there but it's not as prevalent as Windows lessons.  The 2017 Mac version is closer than ever to the Windows version so if you are running 2017 you should be a little better.

I'm going to be blunt with this... Forums are not a place to come in and ask questions like "How do I draw a line?" or How can I dimension?"  You should seek help in the forums after you have looked at instructional materials and you have problems after attempting things based on fundamentals.

It's an opinion and it's only mine.  Please don't take offense.  Take an opportunity to view some content and try to word your questions a bit more pointed.

Good luck in your study.