Before you start designing decals we must cover the basics of Inkscape.
-The Draw Bezier and Straight Lines
After selecting this tool you want to left click to create a node and it'll take a couple nodes to create a rough outline of your shape. You may notice a red line this is to help you create your desired shape.
-Edit Paths by Nodes
Will help you refine your rough outline into the desired shape. Simply click Edit paths by nodes then select your outline and push or pull a line segment till it's correct.
To move nodes around simply click edit paths by nodes. Select your outline,left click and hold on a node then drag your line.
Select the paint brush located on the top tool. A window should open and select stroke style. Make sure to click on your line. Topically 2.000 and 3.000 PX are the stranded line widths I use for all my decals.
-Tools before we start.
Mini Figure Template
Lego Color Guild
If you require any more help feel free to P.M me or leave a reply.