AWS by default does not provide memory usage statistics for EC2 Instance. You'll need to configure Cloudwatch to monitor these system-level matrix.

The entire process just has 4 steps, however, it seems complicated at first. Breaking it down for you.

1. Create a IAM role with CloudWatchFullAccess permission
2. Attach this IAM role to your EC2 instances
3. Download and install the Cloudwatch agent
4. Configure the Cloudwatch agent

Let's see them in detail.

Step 1: Create a role name - AWSCustomRoleForEC2 with CloudWatchFullAccess permission. Assign the above role to your running AWS EC2 instance.

Step 2: Download and install the Cloudwatch agent in your EC2 instance

wget https://s3.amazonaws.com/amazoncloudwatch-agent/ubuntu/amd64/latest/amazon-cloudwatch-agent.deb

sudo dpkg -i -E ./amazon-cloudwatch-agent.deb

Step 3: Launch the Cloudwatch wizard

sudo /opt/aws/amazon-cloudwatch-agent/bin/amazon-cloudwatch-agent-config-wizard

Choose these settings

=============================================================
= Welcome to the AWS CloudWatch Agent Configuration Manager =
=============================================================
On which OS are you planning to use the agent?
1. linux
2. windows
default choice: [1]:

Trying to fetch the default region based on ec2 metadata...
Are you using EC2 or On-Premises hosts?
1. EC2
2. On-Premises
default choice: [1]:

Which user are you planning to run the agent?
1. root
2. cwagent
3. others
default choice: [1]:

Do you want to turn on StatsD daemon?
1. yes
2. no
default choice: [1]:

Which port do you want StatsD daemon to listen to?
default choice: [8125]

What is the collect interval for StatsD daemon?
1. 10s
2. 30s
3. 60s
default choice: [1]:
3

What is the aggregation interval for metrics collected by StatsD daemon?
1. Do not aggregate
2. 10s
3. 30s
4. 60s
default choice: [4]:
4

Do you want to monitor metrics from CollectD?
1. yes
2. no
default choice: [1]:
2

Do you want to monitor any host metrics? e.g. CPU, memory, etc.
1. yes
2. no
default choice: [1]:

Do you want to monitor cpu metrics per core? Additional CloudWatch charges may apply.
1. yes
2. no
default choice: [1]:
2

Do you want to add ec2 dimensions (ImageId, InstanceId, InstanceType, AutoScalingGroupName) into all of your metrics if the info is available?
1. yes
2. no
default choice: [1]:
2

Would you like to collect your metrics at high resolution (sub-minute resolution)? This enables sub-minute resolution for all metrics, but you can customize for specific metrics in the output json file.
1. 1s
2. 10s
3. 30s
4. 60s
default choice: [4]:
4

Which default metrics config do you want?
1. Basic
2. Standard
3. Advanced
4. None
default choice: [1]:

Current config as follows:
{
    "agent": {
        "metrics_collection_interval": 60,
        "run_as_user": "root"
    },
    "metrics": {
        "metrics_collected": {
            "disk": {
                "measurement": [
                    "used_percent"
                ],
                "metrics_collection_interval": 60,
                "resources": [
                    "*"
                ]
            },
            "mem": {
                "measurement": [
                    "mem_used_percent"
                ],
                "metrics_collection_interval": 60
            },
            "statsd": {
                "metrics_aggregation_interval": 60,
                "metrics_collection_interval": 60,
                "service_address": ":8125"
            }
        }
    }
}

Are you satisfied with the above config? Note: it can be manually customized after the wizard completes to add additional items.
1. yes
2. no
default choice: [1]:

Do you have any existing CloudWatch Log Agent (http://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/AgentReference.html) configuration file to import for migration?
1. yes
2. no
default choice: [2]:

Do you want to monitor any log files?
1. yes
2. no
default choice: [1]:
2

Saved config file to /opt/aws/amazon-cloudwatch-agent/bin/config.json successfully.

Step 4: Start the Cloudwatch agent

sudo /opt/aws/amazon-cloudwatch-agent/bin/amazon-cloudwatch-agent-ctl -a fetch-config -m ec2 -s -c file:/opt/aws/amazon-cloudwatch-agent/bin/config.json

Tip: Command to check status of the Cloudwatch agent

service amazon-cloudwatch-agent status

References:

How to monitor memory of your EC2 instance using Cloudwatch - Youtube

Custom application metrix on AWS using Cloudwatch - Medium