How do I not Accidently Strawman my opponents when steelmanning them?

Steel-manning is taking your opponents argument and making it as strong as possible before proceeding to refute it.
The problem with this is since you are changing your opponent argument, it easy to straw man without realizing it.

Reiterate the steel man argument step by stepp and ask if they agree with it step by step. If they disagree with a change you have made, point out the reason why you made that change and then discuss that change.

Though I have never heard the phrase "steel manning" I use it quite a bit. I don't like to try to win aan argument because of a blunder on my oppoenents part, but just simply seek to develop the strongets ideas possible. There have been important times that i have realized that an oppoenent of mine was right, but for the wrong reasons. Further, "steel manning" helps develop your ability to counter better opponents.

If you were to grant them certain assumptions that they didn't imply, for example assume one of their claims is true where they didn't imply it, and are still able to refute their argument, this wouldn't be committing a strawman.

This is pretty effective when used right, if they are willing to reason.

The key problem with the Strawman is the fact that you're not having an honest discussion with the other person.
It could be described as a: Rhetorical tool, Diversion, or simply miscommunication/misunderstanding.

However, If you sincerely seek to understand the the other person it is a different case.
I'd combine the two in the following way:

1.) Make sure you really understand the other person. Verify that all your assumptions about the other person's views are correct. (avoid Strawman)
2.) When you understand it, see if you could improve (Steelman) the position even further.
3.) Refute the improved argument.

Conclusion: You present the case as the other person sees it (Communication works), after that
you're improving it (Steelman), and then you refute it (Win!)