Here’s how to create a Fire Shader.
As always, you can also find below the HLSL Shader and, if you’re supporting me on Patreon, the download link for the Unity Package.

HLSL

//@febucci, https://www.febucci.com/tutorials/
//Support my work here: https://www.patreon.com/febucci
 
Shader "Fire" 
{
	Properties
	{
		_NoiseTex("Noise Texture", 2D) = "white" {}
		_GradientTex("Gradient Texture", 2D) = "white" {}
 
		_BrighterCol("Brighter Color", Color) = (1,1,1,1)
		_MiddleCol("Middle Color", Color) = (.7,.7,.7,1)
		_DarkerCol("Darker Color", Color) = (.4,.4,.4,1)
	}
 
	SubShader
	{
		//The shader is transparent
		Tags
		{ 
			"RenderType" = "Transparent" 
		}
			
		Blend SrcAlpha OneMinusSrcAlpha
 
		Pass 
		{
 
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc"
			#include "UnityShaderVariables.cginc" //to use _Time
 
 
			sampler2D _NoiseTex;
			sampler2D _GradientTex;
 
			float4 _BrighterCol;
			float4 _MiddleCol;
			float4 _DarkerCol;
 
			//Input for the vertex
			struct appdata {
				float4 vertex : POSITION;
				float4 texcoord : TEXCOORD0;
			};
 
			//Output for the fragment
			struct v2f {
				float4 pos : SV_POSITION;
				float2 uv : TEXCOORD0;
			};
 
			v2f vert(appdata v) {
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);
				o.uv = v.texcoord.xy;
 
				return o;
			}
 
			float4 frag(v2f IN) : SV_Target {
				
				float noiseValue = tex2D(_NoiseTex, IN.uv - float2(0, _Time.x)).x; //fire with scrolling
				float gradientValue = tex2D(_GradientTex, IN.uv).x;
				
				float step1 = step(noiseValue, gradientValue);
				float step2 = step(noiseValue, gradientValue-0.2);
				float step3 = step(noiseValue, gradientValue-0.4);
 
				//The entire fire color
				float4 c = float4
					(
						//Calculates where to place the darker color instead of the brighter one
						lerp
						(
							_BrighterCol.rgb,
							_DarkerCol.rgb,
							step1 - step2 //Corresponds to "L1" in my GIF
						),
 
					step1 //This is the alpha of our fire, which is the "outer" color, i.e. the step1
					);
 
				c.rgb = lerp //Calculates where to place the middle color
					(
						c.rgb,
						_MiddleCol.rgb,
						step2 - step3 //Corresponds to "L2" in my GIF
					);
 
				return c;
			}
			ENDCG
		}
	
	}
}

Amplify Shader