You can use HTML markup or a JavaScript factory function.
Create HTML markup passing in the desired username and a widget ID you have configured on Twitter.com:
<a class="twitter-timeline"
data-widget-id="{widget_id}"
href="https://twitter.com/{username}"
data-screen-name="{username}">
Tweets by @{username}
</a>
Or dynamically create the timeline with a JavaScript factory function:
twttr.widgets.createTimeline(
{widget_id},
document.getElementById({target_element}),
{
screenName: {username}
}
);