You could use Yes Paste. Roll the paste out evenly on the rag board with a three inch wide, short nap paint touch up roller. Cover the wet paste with a sheet of smooth Kraft paper, remove the Kraft paper immediately. This will leave an almost invisible layer of paste on the rag board. Lay your silk onto the paste, cover with Kraft or release paper. Put it in a cold vacuum press for about five minutes. Remove it, keep it flat for a few hours. Your done.
There is an old oriental method using starch paste, very similar to above that has been used for hundreds of years, I use Yes Paste though.
I mount a lot of oriental watercolors on silk this way for several artists. Been doing it for these same folks for about twenty years.
John