Hi !
I personnally paint all my markings 'cause I hate decals, and I play Ultramarines too.
I tried several solutions and now I prefer to paint the symbol itself directly rather than paint a white square or circle and detail away with blue.
I start with astronomican grey from the foundation range, but I think there is a white base now so this may help. I draw reference points that I'll use to place the marking. Then I draw lines to have a general shape. Next, I thicken those lines/fill the shapes.
At this step, I correct with blue the inevitable mistakes that I have made.
And then I repaint the marking with white (this is where the white base could help, to avoid this step).
I like to use different markings, I use the Insignum Astartes artbook as a reference. For a crux terminatus symbol I proceed as follow :
- 4 dots, up, down, left, right
- two thin lines to have a vertical cross
- 4 short segments on the ends of both lines, now the symbol looks like 4 "T"s joined on the center of the shoulder pad by their "bases"
- I join the ends of the segments to the center. Now I have 4 triangles.
- I fill the triangles
- Corrections with blue
- Repaint with white