The Focal Manipulator allows you to craft a Focus that you can use with your Caster’s Gauntlet. If you put it in your offhand, you will see icons on the top left of your screen. The big circle is the Focus you currently have equipped (none at the moment), and the bar is how much Vis is in the area (each chunk has a different amount of Vis).
You have already crafted a Blank Lesser Focus. You can use the Focal Manipulator to add some effects. If you put the Lesser Focus into the Focal Manipulator you will see a few options:
Just like an Enchanting Table, you can rename this focus at the top. On the right you can see the total complexity (0/15), the XP cost (vanilla experience) and the Vis cost. If yours looks different, try removing the focus and inserting it again.
On the left you can see two icons: Touch and Fire. You can add these to the Focus by clicking them:
In this example, when you touch something it will be set on fire. You can edit the duration and the power of the flame. With Touch there are no options. The longer the duration, and the higher you set the power, the more this will cost, and the higher the complexity will be. If the complexity is too high, you will not be able to craft this Focus. You have to balance this out.
Note: If you set the Burn Duration to 0 it will not set things or mobs on fire.
You will also see Vis per cast. This Focus will cost 0.8 Vis to cast and casting it too much might drain the area of Vis. There is also a Crystal requirement to craft it.
Though you may be tempted to make a Touch focus, a Bolt focus will serve you much better if you have the patience for it.
Once crafted, put the Focus into your inventory and press and hold F (or the appropriate key binding). This allows you to select what Focus you would like to put into your Gauntlet. Sneak + F will remove the focus.
Once you have selected it you will see the top left of your screen has a red circle. Sneaking will tell you how much the spell costs and how much vis is in the area:
If you used the touch medium, you can right click to spew some fire at a range of 4 blocks.
If you spam this it will slowly use all the Vis in the area. You can redo your focus by inserting into the Focal Manipulator. You can set it to not burn if you want to avoid forest fires.