The Assembler is used to craft and upgrade many items and blocks in the Environmental Mods. It is also one of the first machines you will need to craft in order to progress in any of the Environmental Mods.
In the Assembler the order in which each slot is populated is: top, bottom, left, right, center.
Modifiers/Speeding up the Assembler
The Assembler is pretty slow by default but you can make it a lot faster by using Frequency Modifiers(Yes the same ones you put in your Void Miner). Using the bottom 4 slots in the GUI you can have up to 16 modifiers installed at a time.
It is pretty simple to automate the Assembler. The button in the bottom left of the GUI switches the input slots between accepting 64 items or 1 item per slot. Make sure this is set to 1 item per slot if you intend on only crafting 1 thing at a time.
If you are using Refined Storage its best to use its JEI + button to create the recipes as it puts the input items in the right insertion order in the pattern.