Here I will explaing how to create custom paymnet method in Magento 2.

Follow the below steps.

Declare Module

File : app/code/PHPCode/Quickpay/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noTestSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
 <module name="PHPCodez_Quickpay" setup_version="2.0.0" active="true">
 </module>
</config>

Register the module

File : app/code/PHPCodez/Quickpay/registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
 \Magento\Framework\Component\ComponentRegistrar::MODULE,
 'PHPCodez_Quickpay',
 __DIR__
);

Create config.xml

File : app/code/PHPCodez/Quickpay/etc/config.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../Store/etc/config.xsd">
 <default>
 <payment>
 <quickpay>
 <payment_action>capture</payment_action><!-- You can use another methor like capture -->
 <model>PHPCodez\Quickpay\Model\PaymentMethod</model>
 <active>1</active>
 <title>PHPCodez Quickpay</title>
 <order_status>pending</order_status><!-- set default order status-->
 </quickpay>
 </payment>
 </default>
</config>

Create system configuration file

File : app/code/PHPCodez/Quickpay/etc/adminhtml/system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
 <system>
 <section id="payment">
 <group id="quickpay" translate="label" sortOrder="2" showInDefault="1" showInWebsite="1" showInStore="1">
 <label>PHPCodez Quick Pay</label>
 <field id="active" translate="label comment" sortOrder="1" type="select" showInDefault="1" showInWebsite="1" showInStore="0">
 <label>Enable</label>
 <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
 </field>
 </group>
 </section>
 </system>
</config>

Create model file to define payment

File : app/code/PHPCodez/Quickpay/Model/PaymentMethod.php

<?php
 namespace PHPCodez\Quickpay\Model;
 class PaymentMethod extends \Magento\Payment\Model\Method\AbstractMethod {
 protected $_code = 'quickpay';
 }

Create method-renderer.js

File : app\code\PHPcodez\Quickpay\view\frontend\web\js\view\payment

define(
 [
 'uiComponent',
 'Magento_Checkout/js/model/payment/renderer-list'
 ],
 function (
 Component,
 rendererList
 ) {
 'use strict';
 rendererList.push(
 {
 type: 'quickpay',
 component: 'PHPCodez_Quickpay/js/view/payment/method-renderer/quickpay'
 }
 );
 return Component.extend({});
 }
);

Create quickpay.js

File : app\code\PHPcodez\Quickpay\view\frontend\web\js\view\payment\method-renderer\Quickpay.js

define(
 [
 'Magento_Checkout/js/view/payment/default'
 ],
 function (Component) {
 'use strict';
 
 return Component.extend({
 defaults: {
 template: 'PHPCodez_Quickpay/payment/quickpay'
 }
 });
 }
);

Create template file quickpay.html

File : app\code\PHPcodez\Quickpay\view\frontend\web\template\payment\quickpay.html

<div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked())}">
 <div class="payment-method-title field choice">
 <input type="radio"
 name="payment[method]"
 class="radio"
 data-bind="attr: {'id': getCode()}, value: getCode(), checked: isChecked, click: selectPaymentMethod, visible: isRadioButtonVisible()"/>
 <label data-bind="attr: {'for': getCode()}" class="label"><span data-bind="text: getTitle()"></span></label>
 </div>
 <div class="payment-method-content">
 <!-- ko foreach: getRegion('messages') -->
 <!-- ko template: getTemplate() --><!-- /ko -->
 <!--/ko-->
 <div class="payment-method-billing-address">
 <!-- ko foreach: $parent.getRegion(getBillingAddressFormName()) -->
 <!-- ko template: getTemplate() --><!-- /ko -->
 <!--/ko-->
 </div>
 <div class="checkout-agreements-block">
 <!-- ko foreach: $parent.getRegion('before-place-order') -->
 <!-- ko template: getTemplate() --><!-- /ko -->
 <!--/ko-->
 </div>
 <div class="actions-toolbar">
 <div class="primary">
 <button class="action primary checkout"
 type="submit"
 data-bind="
 click: placeOrder,
 attr: {title: $t('Place Order')},
 css: {disabled: !isPlaceOrderActionAllowed()},
 enable: (getCode() == isChecked())
 "
 disabled>
 <span data-bind="i18n: 'Place Order'"></span>
 </button>
 </div>
 </div>
 </div>
</div>

Create checkout_index_index.xml

File : app\code\PHPcodez\Quickpay\view\frontend\layout\checkout_index_index.xml

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
 <body>
 <referenceBlock name="checkout.root">
 <arguments>
 <argument name="jsLayout" xsi:type="array">
 <item name="components" xsi:type="array">
 <item name="checkout" xsi:type="array">
 <item name="children" xsi:type="array">
 <item name="steps" xsi:type="array">
 <item name="children" xsi:type="array">
 <item name="billing-step" xsi:type="array">
 <item name="component" xsi:type="string">uiComponent</item>
 <item name="children" xsi:type="array">
 <item name="payment" xsi:type="array">
 <item name="children" xsi:type="array">
 <item name="renders" xsi:type="array">
 <!-- merge payment method renders here -->
 <item name="children" xsi:type="array">
 <item name="quickpay" xsi:type="array">
 <item name="component" xsi:type="string">PHPCodez_Quickpay/js/view/payment/method-renderer</item>
 <item name="methods" xsi:type="array">
 <item name="quickpay" xsi:type="array">
 <item name="isBillingAddressRequired" xsi:type="boolean">true</item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </item>
 </argument>
 </arguments>
 </referenceBlock>
 </body>
</page>

Issue the following commands and you will be able to see payment method ‘PHPCodez Quick Pay’ on checkout page.

php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy -f
php bin/magento cache:clean