I find a nice stiff jab (while mostly an arm punch) involves leaning forward and pushing it out starting with your shoulder. A forward step may be involved but not necessary.
If you are stepping too far out, you sacrifice some balance and power.
like so many other techniques - it's honestly so much easier to just practice repetitiously on a punching bag. Reading the best & most accurate advice will just confuse you and make you focus on things more than necessary. Honestly, most things you just have to learn purely by feel.