This is an example weaver componet

ExampleWeaverComponent.cs

using System;
using Mono.Cecil;
using UnityEngine;
using Mono.Cecil.Cil;

namespace Weaver
{
    // Inherit WeaverComponent to get callbacks and to show up as a componet in our ScriptableObject settings. 
    public class ServerCommandComponent : WeaverComponent
    {
        // Used for logging the name really does not matter. 
        public override string addinName
        {
            get
            {
                return "Server Command";
            }
        }

        // Defines which type of callbacks you want. Used as an optimization 
        public override DefinitionType effectedDefintions
        {
            get
            {
                return DefinitionType.Method;
            }
        }

        // Invoked for every method in each assembly that is listed in WeavedAssemblies in settings. 
        public override void VisitMethod(MethodDefinition methodDefinition)
        {
             CustomAttribute serverCommandAttribute = methodDefinition.GetCustomAttribute<ServerCommandAttribute>();

            if (serverCommandAttribute == null)
            {
                // Our method does not have the attribute so we skip it. 
                return;
            }

            // Do the IL Injection. 
        }
    }
}