So, you're asking specifically for what method to use to apply the grass, but in my eyes there's nothing wrong with how you've applied yours. If I interpret your question more broadly it sounds like "give me tips to improve the basing", and to that my first tip would be to involve more elements.
Let me illustrate, your bases involve one thing — grass of one shade on the perfectly flat base top. I did an exercise to improve my basing recently and in the end I have more than ten things going on with my basing. If that sounds daunting (and boastful), don't worry
. It's not that complicated. And you could skip a lot of my steps and still get something more visually complex than what you currently have.
For example, using a grit mix of sand, tiny ballast and coarser ballast glued to the base, painted dark brown and drybrushed medium brown before applying the grass on top in a more random and patchy manner. Neither very challenging nor time consuming and your basing now involves several elements to create visual interest; the texture and colour of both dirt and grass and the distribution thereof on gently undulating ground.
Many ways to go from here — ground shades, grass shades, cork, bark, plaster, twigs, grit, tufts, lichen, moss, coffee granules, tea laves, dried spices and much more in terms of sheer building materials.
Additionally, don't forget that you're the artist and the director of your little mini art piece. What I mean by that is that a selection of materials will be used no matter what you do, but applying them like a robot and trusting in the recipe to always deliver the result may limit you. My first component list (brown dirt, green grass) can be used for very sparse grass on muddy ground, or almost all-encompassing heavy grass coverage with the odd mud crack. You can vary the ratio on the bases, or go even further and let only some bases show the anomaly. Exactly the same components but with a big difference in execution and result.
Your vision and the context of your game environment and/or backstory can be your vehicles towards a nice basing scheme. For example, the army I'm building at the moment is one I envision to be campaigning in the hinterlands and rural areas of the Empire in damp weather. This has guided me in defining the basing scheme; the grass shade is deep lush green because the environment is quite humid, accented by the odd paler tuft on a raised feature, away from the soggy ground. The odd stone, boulder and log are embedded in the ground because the vegetation and climate would build up silt around such features. I've painted these features mixing in browns and greens to represent the discoloration that dirt and organic growths cause in that environment. I have a lot of lumps and bumps in the ground. If I swapped out the dirt and grass for paler shades, applied less grass, left the rubble more exposed and built flatter ground bits it would look more like an arid windswept plain with sparse vegetation because that's the character dirt, grass and ground take on under such circumstances.
Do you have a particular idea about what you would like to achieve with the basing? Or on the contrary, open to any suggestions as long as the results looks "better" than what you have? "Better" can entail so many things, it all depends on what look you're after to a high degree.