I just received an order from these guys. Nice stuff.
You can order the "Kit" or just the stencil. Here's the kit:
https://www.proto-advantage.com/store/product_info.php?products_id=3410015
The "stencil" was on the related products for the page. The videos show how to use their stuff.
In any event, SMT boards are soldered by applying a "solder paste" through a mask, a.k.a, the "stencil". The mask can be metal or mylar. The stencil generally covers the entire PCB. Then the components are placed and set in an oven with a specific Temperature vs Time profile.
In double-sided SMT boards, the components on the bottom might be held on by surface tension alone or a glue is applied, There is a low temperature curing step required for the glue. The glue is applied with a nozzle.
The solder paste has a limited shelf life for stencil applications.
So, components like your connector with pins on the bottom require hot air to solder manually.
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The SMD headers use an alternating pattern. The kit contains a small PCB with holes to set the separation of the headers while soldering. The PCB separator is not used after soldering.