Do it from ground up like normal

by copying a tutorial/demo

show_simulation.gdshader

@@ -0,0 +1,7 @@
+shader_type canvas_item;
+uniform sampler2D trail_tex;
+
+void fragment() {
+	float v = texture(trail_tex, UV).r;
+	COLOR = vec4(vec3(v), 1.0);
+}