I currently think the easiest medium to use is the Liquitex Ultra Matte Gel.
It is a little more pricey but it is a big jar and so you get a lot to use. I have tried the Golden Super Matte Medium and it works well also but I tend to think now that the Liquitex is a little more matte. The Golden is not as thick as the Liquitex so that is another difference which you might want to consider. When I am using the Liquitex I typically mix abougt 3 drops of Golden Fluid Paints + 1 small brush full or large english pea sized amount of the Liquitex Ultra Matte Gel + 20-30 drops Distilled water. If I am needing more working time I will add 2-3 drops of either Golden or Jo Sonjas Retarder. Once I have my paint mixed up, I adjust my water by adding more water or a tad more paint depending on whether I need my pigment strength stronger or lesser. If I find the paint is beading up on the vinyl when applied (this comes from too high ratio of water in the paint usually), I add just a little more of the Liquitex Ultra Matte Gel. This formula pretty much has worked for me with all the paint brands I have tried thus far - Golden, Jo Sonjas, RebornFX, and Miracle Blend. Although the RebornFX and Miracle Blend paints have some mediums in them already, I find if I am wanting to thin down the RebornFX adding the mediums in works to balance the addition of the water. WIth Miracle Blend I found their paints are more proned to bead up on the vinyl and some have reported fading with over thinning. Thus adding in the mediums controls all that. I have not tried adding mediums in with the Ultimate Fusion but I have read some say they have done this with success. I tested the Ultimate Fusion using their water mix method and it works ok but I found they tend to bead up a lot as well and muddy around when thinned a lot so adding in mediums would make sense to me.
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