Just flare the tubing as if for a standard connection.

Steel tubing is often difficult to flare well enough to seal tightly, especially after multiple openings. Copper seats are available commercially but often hard to find. Making useable ones is easy. Just flare a soft copper tube of the appropriate size and carefully cut the flare off as a seal for the steel tube. If you have a flat copper washer of the proper size, it can sometimes be used without using this technique.