Use some strong tasting herbs such as thyme, sage, and bay-leaf and for the spices use pounded coriander seed, allspice berries and black pepper. Avoid ready-ground spices.
You can use black or white mustard seed, or a mixture.
Soak it for a couple of days. Pour off any excess vinegar.
Use a food process unless you are feeling very energetic, have not got a pestle and mortar or want a very grainy mustard.
Taste it and see if you want to add salt, pepper, honey or a liquid such as apple juice or more vinegar.
Store it in glass jars with plastic lids which will not corrode.
I found this recipe in an old manuscript while I was at university.
Notes
Use wine or cider vinegar. Heating it helps absorb the flavours of the herbs and spices.