You want to make a solution of vinegar and steel wool which is known as ferric acetate or iron(III) acetate.
Just take a glass jar and fill completely with wire wool and cover with vinegar. Do not put a tight fitting lid on. Let this sit for 3-5 days until there are no more bubbles forming (hydrogen gas). This is a sign that the chemical reaction is finished. Sieve the finished black solution through some cheese cloth or similar into a clean glass jar.
You can experiment with different strengths by thinning some of the solution down with vinegar. Always make test strips first before applying to your model.
Different woods react differently. The more tanin in the wood the darker the results will be.
It has been used in furniture making for years to emulate ebony.
Hope this helps.