public Item CreatePartialDesign(string partialDesignName)
{
    // Get the parent item
    var masterDb = Sitecore.Configuration.Factory.GetDatabase("master");
    var parentItem = masterDb.Items["/sitecore/content/My Tenant/My Site/"];

    //Now we need to get the template from which the item is created
    TemplateItem partialDesignTemplate = masterDb.GetTemplate("Project/My Tenant/Partial Design");

    //Now we can add the new item as a child to the parent
    return parentItem.Add(partialDesignName, partialDesignTemplate);
}

var partialDesignItem = CreatePartialDesign("New Partial Design");
string renderingXml = sampleItem["__Renderings"];
LayoutDefinition layoutDefinition = new LayoutDefinition();
layoutDefinition.LoadXml(renderingXml);

string defaultDeviceId = "{FE5D7FDF-89C0-4D99-9AA3-B5FBD009C9F3}";
DeviceDefinition deviceDefinition = layoutDefinition.GetDevice(defaultDeviceId);
string sampleLayoutId = "{layout-item-guid}";
deviceDefinition.Layout = sampleLayoutId;

// Add any renderings you want to a bit like this. You will need the rendering Id's
string sampleRenderingId = "{rendering-item-guid}";
RenderingDefinition renderingDefinition = new RenderingDefinition();
renderingDefinition.ItemID = sampleRenderingId;
renderingDefinition.Placeholder = "content";
deviceDefinition.AddRendering(renderingDefinition);

// Write the presentation back to the renderings field
string outputXml = layoutDefinition.ToXml();
sampleItem.Editing.BeginEdit();
sampleItem["__Renderings"] = outputXml;
sampleItem.Editing.EndEdit();