There are a few things I can think of that would help:
- Sprinkle the salt over the surface rather than dumping a blob in one place.
- Add the salt in multiple additions, stirring between each.
- Stir sufficiently to distribute.
I made pancakes a few months ago and forgot the salt. After eating the first couple, I realized the mistake and added salt to attempt to give it some flavor. Unfortunately, I didn't follow the three points above and ended up with salty patches in my pancakes, so some pancakes were still unsalted and others were so salty it (literally) made me feel like I was going to throw up.
Other than promising myself to never wait so late to add the salt in the future, I thought about solutions and came up with the points I mentioned.
If you add the salt to one spot in the mixture, it will be much more difficult to evenly disperse the salt throughout. So, sprinkling it on the surface makes it less likely there will be clumps of overwhelming saltiness.
With something like mashed cassava or potatoes, you can taste for saltiness, so adding small amounts and tasting between additions is a great way to be sure you get the level of salt you want and since you stir between the additions, the part of the mixture that the salt is applied to, is likely to be different with the subsequent additions.
Stirring sufficiently is the final point... if you only stir once or twice, it's unlikely that the salt will reach all points in the mixture, likely remaining towards the top. Be certain to stir enough and do it well, being sure to pull the mixture from the bottom to fully incorporate.