How to butt weld 2 pipes together straight?

All of the pipe welding advice is good , grinding the ends to be welded at a slight angle and keep a small gap between the pipes . Clamp two pieces of angle , one on each side . I use heavy gauge angle and small enough to give me a decent area to tack , then I turn the pipe to make sure it is still straight and tack on the areas that were covered by the angle previously . Don't rush it , let it cool a bit before you reposition it in the angle .
To fit pipe inside of pipe find a supplier that stocks mechanical tubing and get a chart.
Look at the wall thickness , OD, and ID . You can get a pretty good fit as a rule .
If you can't then get a piece that is slightly over the ID of the tube you are working with and turn it down to fit or have it turned if you don't have the equipment .
Good luck with your project .