how can i blend textures on a specific place on another texture and change it's alpha value?

I am trying to do a splash effect when an object is hit.

I've been playing around with shaders and got to a point that i can blend two textures on a position determined by inspector. But i can't change it's alpha value no matter what and i see no way of adding a new texture on different place.

Anyone could help?

Here's my shader code if that helps (i don't really understand it :D )

 Shader "Examples/2 Alpha Blended Textures" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Color ("color", Color) = (1,1,1,0.5)
         _BlendTex ("Alpha Blended (RGBA) ", 2D) = "white" {}
         
     }
     SubShader {
                 Tags { 
                     "Queue"="Transparent" 
                     "IgnoreProjector"="True" 
                     "RenderType"="Transparent" 
                     "PreviewType"="Plane"
                     "CanUseSpriteAtlas"="True" 
                 }
                 Lighting Off
                 ZWrite Off
                 Cull Back
                 Blend SrcAlpha OneMinusSrcAlpha
                  Material {
                     Emission [_Color]
                     Diffuse  [_Color]
                  }
              Pass {
                 SetTexture [_BlendTex] {
                     constantColor[_Color]
                     combine constant * texture // this thing doesn't work here
                     combine texture lerp (texture) previous //but when theese two lines are swapped, the alpha works but the base image disapears
                 }
             } 
      }
  }