[chore]: update relay implementation

smart-contracts/advanced/relay.md

@@ -41,6 +41,74 @@ Transactions can be paid for in two primary ways: off-chain payments and on-chai
 
 - **callWithSyncFee**: This method is similar to callWithSyncFeeERC2771 but without the need for ERC-2771’s off-chain signature verification. The user’s gas fee is calculated and paid directly from the target smart contract during the transaction execution. This approach is useful for applications where users are expected to pay for their own gas without requiring meta-transaction features.
 
+### Implementation
+
+We will require three simple steps to implement Gelato Relay. Here, we are going to showcase the three steps required to implement the method `sponsoredCallERC2771`, which is the most used one.
+
+#### Step 1: Inherit Context Contract
+
+Depending on the method, you must inherit different contracts as they will provide other methods. In this case, we will have to inherit the `ERC2771Context`. The `ERC2771Context` provide us with the methods `_msgSender()` and `_msgData()` that will allow us to recover the original user sending the transaction.
+
+```solidity
+import {
+    ERC2771Context
+} from "@gelatonetwork/relay-context/contracts/vendor/ERC2771Context.sol";
+
+contract CounterERC2771 is ERC2771Context {
+
+    // ERC2771Context: setting the immutable trustedForwarder variable
+    constructor(address trustedForwarder) ERC2771Context(trustedForwarder) {}
+
+    function incrementContext() external {
+
+        // Incrementing the counter mapped to the _msgSender!
+        contextCounter[_msgSender()]++;
+
+        // Emitting an event for testing purposes
+        emit IncrementContextCounter(_msgSender());
+    }
+}
+```
+
+#### Step 2: Import the relay SDK
+
+In your frontend/backend, you would need to import and instantiate the relay class.
+
+```
+import { GelatoRelay, SponsoredCallERC2771Request } from "@gelatonetwork/relay-sdk";
+const relay = new GelatoRelay(API_KEY);
+```
+
+#### Step 3: Send the payload to Gelato
+
+This is an example using Gelato's CounterERC2771.sol, which is deployed on these networks.
+
+```
+// Set up on-chain variables, such as target address
+const counter = "0x00172f67db60E5fA346e599cdE675f0ca213b47b";
+const abi = ["function incrementContext()"];
+const provider = new ethers.BrowserProvider(window.ethereum);
+const signer = provider.getSigner();
+const user = signer.getAddress();
+
+// Generate the target payload
+const contract = new ethers.Contract(counter, abi, signer);
+const { data } = await contract.incrementContext.populateTransaction();
+
+// Populate a relay request
+const request: CallWithERC2771Request = {
+  chainId: (await provider.getNetwork()).chainId,
+  target: counter;
+  data: data;
+  user: user;
+};
+
+// Without a specific API key, the relay request will fail!
+// Go to https://relay.gelato.network to get a testnet API key with 1Balance.
+// Send a relay request using Gelato Relay!
+const relayResponse = await relay.sponsoredCallERC2771(request, provider, apiKey);
+```
+
 #### Further Gelato resources
 
 - [Gelato Relay Docs](https://docs.gelato.network/web3-services/relay)