LWCModal.html

<template>
    <div class="slds-theme_default">
        <div class="slds-p-around_medium">
            <lightning-button label="Show LWC Modal" variant="brand" onclick={handleOpenModal}>
            </lightning-button>
        </div>
        <template if:true={isOpenModal}>
            <div style="height: 500px;">
                <section role="dialog" tabindex="-1" aria-labelledby="modal-heading-01" aria-modal="true" aria-describedby="modal-content-id-1" class="slds-modal slds-fade-in-open">
                    <div class="slds-modal__container">
                        <header class="slds-modal__header">
                            <button class="slds-button slds-button_icon slds-modal__close slds-button_icon-inverse" title="Close" onclick={handleCloseModal}>
                                <lightning-icon icon-name="utility:close" variant="inverse" alternative-text="Close" size="medium">
                                </lightning-icon>
                                <span class="slds-assistive-text">Close</span>
                            </button>
                            <h2 id="modal-heading-01" class="slds-text-heading_medium slds-hyphenate">LWC Modal Example</h2>
                        </header>
                        <div class="slds-modal__content slds-p-around_medium" id="modal-content-id-1">
                            <div class="slds-text-heading_small slds-text-align_center">
                                Welcome to Biswajeet Samal's Blog
                            </div>
                        </div>
                        <footer class="slds-modal__footer">
                            <lightning-button label="Cancel" variant="neutral" onclick={handleCloseModal}></lightning-button>
                        </footer>
                    </div>
                </section>
                <div class="slds-backdrop slds-backdrop_open"></div>
            </div>
        </template>
    </div>
</template>

LWCModal.js

import {LightningElement, track} from 'lwc';

export default class LWCModal extends LightningElement {
  
    @track isOpenModal = false;

    handleOpenModal() {
        this.isOpenModal = true;
    }
  
    handleCloseModal() {
        this.isOpenModal = false;
    }
}

LWCModal.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="LWCModal">
    <apiVersion>46.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__AppPage</target>
        <target>lightning__RecordPage</target>
        <target>lightning__HomePage</target>
    </targets>
</LightningComponentBundle>

Output: