Regalrez 1126

The easiest is to purchase regalrez varnish that contains a matting agent. A Google search showed that it can be purchased in gloss, satin, and matte.
Some make their own by adding small amounts of microcrystine wax. This is best done by melting the wax on a hot plate and adding it to the varnish well away from the heat source. You can then heat up a pan of water, remove it from the heat source and place the jar of varnish containing the wax if you need more heat to encourage complete solution. Make sure that the whole operation is done in a place of very good ventilation and where there is no open flame or source of high heat. The varnish will remain cloudy in solution and some wax may precipitate. Personally, I have never been very happy with this method.
I prefer to just use lower resin concentration solutions and the keep brushing as the varnish sets. This is tiring but one can create surfaces with lower sheen.