Most steel cutting used to be carried out with an oxyacetelene torch. Today, cutting is also done with plasma cutters which are similar to a cutting torch. Whether gas or plasma, if you need to cut a straight line or any precise line, you’re probably going to need a guide of some kind. There are several available.
Straight Edge - The simplest kind of guide is a straight edge. Experiment on some scrap. Make a cut using the straight edge as a guide. Measure the distance from the straight edge to the cut. Now you know how much offset your torch has. This kind of straight edge requires clamping, and doesn't help you steady your torch vertically. But it does help cut straight lines.
Round Torch Guides - Round torch guides look similar to hose clamps. They are metal sleeves that slides around steel casing. They have raised edges on their sides for your torch to follow. They come in a variety of diameters. Tighten the correct diameter to your steel casing stock and they will help you cut a straight, perpendicular line.
Roller Guides - Roller guides are a more sophisticated torch guide for cutting flat stock. They have an armature that secures the cutting tip. They are often adjustable in both angle and cutting height. The armature is mounted to a flat rolling base. The wheels track straight along the stock, giving you a straight line without clamping a straight edge.
Guide Machines - Most mechanical replicators have given way to CNC, computer-controlled manufacturing, though guide machines are still sometimes used with cutting torches. The idea is the same as a pantograph. There is an armature with one or more cutting torches. The armature also has a guide. The guide follows a "blank" --- a circle for example --- and the torch moves in exactly the same motion. If there are multiple torches, the guide can cut as many shapes as there are cutting torches in a single contour of the shape.