<!-- lightning button for open modal window -->
    <lightning-button variant="brand"
       label="What is Modal/PopUp in LWC?"
       title="What is Modal/PopUp in LWC?"
       onclick={openModal}
       class="slds-m-left_x-small">
    </lightning-button>
    <!--Use template if:true to display/hide popup based on isModalOpen value--> 
    <template if:true={isModalOpen}>
        <!-- Modal/Popup Box LWC starts here -->
        <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">
                <!-- Modal/Popup Box LWC header here -->
                <header class="slds-modal__header">
                    <button class="slds-button slds-button_icon slds-modal__close slds-button_icon-inverse" title="Close" onclick={closeModal}>
                        <lightning-icon icon-name="utility:close"
                            alternative-text="close"
                            variant="inverse"
                            size="small" ></lightning-icon>
                        <span class="slds-assistive-text">Close</span>
                    </button>
                    <h2 id="modal-heading-01" class="slds-text-heading_medium slds-hyphenate">Modal/PopUp Box header LWC</h2>
                </header>
                <!-- Modal/Popup Box LWC body starts here -->
                <div class="slds-modal__content slds-p-around_medium" id="modal-content-id-1">
                    <p><b>Modals/Popup Box are used to display content in a layer above the app.
                    </b></p>
                    <p><b>This paradigm is used in cases such as the creation or editing of a record, as well as various types of messaging and wizards.
                    </b></p>
                </div>
                <!-- Modal/Popup Box LWC footer starts here -->
                <footer class="slds-modal__footer">
                    <button class="slds-button slds-button_neutral" onclick={closeModal} title="Cancel">Cancel</button>
                    <button class="slds-button slds-button_brand" onclick={submitDetails} title="OK">OK</button>
                </footer>
            </div>
        </section>
        <div class="slds-backdrop slds-backdrop_open"></div>
    </template>

@track isModalOpen = false;
    openModal() {
        // to open modal set isModalOpen tarck value as true
        this.isModalOpen = true;
    }
    closeModal() {
        // to close modal set isModalOpen tarck value as false
        this.isModalOpen = false;
    }
    submitDetails() {
        // to close modal set isModalOpen tarck value as false
        //Add your code to call apex method or do some processing
        this.isModalOpen = false;
    }