Pretty much on top of it.
First start by entering in your water profile by clicking on 'ingredients' > 'water' and then selecting 'add water' from the tool bar. Once you have entered all the information as best you can, save this profile under your preferred name for it.
Next decide on the type of profile you want to use for the recipe. There are many preloaded with the program which are available including John Palmer's profiles based upon beer color and balance, geographical location, or style specific. There are pros and cons to which you select to match. You can also use the water tool to develop your own profile to the mineral and salt balance you choose.
<Editorial note> I do not recommend using geographical profiles, as there is no way you know how the brewers in that region may or may not (a) source that water or (b) pre-treat that water. You can use Palmer's guidelines, but note that these are designed to be built up from RO water, so you may or may not get a good match to these using your own water. The specific style profiles are in kind of the same boat, you don't necessarily know where the base water started from to ensure you will hit it OK. Once you get some experience under your belt and understand what you want to achieve for mineral and salt levels, building your own from your base water will ensure you always are able to hit the targets you make. <end of editorial commentary.>
Now in your recipe, go to the water tab. Add your water and the program will add it to the recipe ingredients in the amount calculated by BeerSmith. Below that, you can see the box for 'Water Adjustment Salts'. You can click on 'match profile' and a pop up window will guide you through selecting the profile you want to hit. The program will then calculate out the mineral salts needed to come close to that profile from your starting water. I highly recommend that you click on the box that states 'exclude chalk' as this water agent has a specific method for getting it into solution which changes the whole procedure of preparing your water.
Alternatively, in the water adjustment salt box you can click and add mineral salts one at a time and manipulate them your self to obtain the mineral profile you want. IMHO, it is much easier to design the profile using the water tool and then save it using a unique name so you can use that in the recipe as the profile to match.
There are several videos which can walk you through this if you want more of a hands on tutorial:
https://www.youtube.com/watch?v=b_RzgzcejGU
https://www.youtube.com/watch?v=MnkTbh3xSeU&t=282s
https://www.youtube.com/watch?v=mJEYk6_HNl4
https://www.youtube.com/watch?v=m2UG9PACezI
Once you are done, any change you make to your equipment profile, water balance or grain bill will affect the amount of water needed and you will need to go through the water tab again to adjust the water to where you want it to be. Given this, I recommend that the water and water agents should be the last thing you adjust in your recipe before printing it out or brewing.
You will also note that the water adjustments have an effect on the mash pH estimation. To understand how this works, Brad has a video at https://www.youtube.com/watch?v=SxqWEo0v70I
Edit: Forgot to add for clarification, when you use the water tab in the recipe to match your water profile, the program will scale the mineral salt additions based upon the amount of water needed for the recipe.