In a nutshell, microRNAs are short oligonucleotides (almost 22 BP) that either increase or decrease the expression of certain genes.

To improve a trait, you must know which genes should be supposedly up- or downregulated. Note that, there can be multiple genes working in harmony that are responsible for a certain trait. So, you may need to target all of those genes. Supposing classical Mendelian inheritance, if only one gene is responsible for your trait and increasing that gene expression brings in the desired trait, you may employ several techniques for that. Since you are interested in miRNA, you should identify the miRNAs that regulates the gene expression transcriptionally or translationally. Of course, you have to go through the literature for it. Once you have the information about the miRNAs, you can transfect the miRNAs responsible for up or downregulation of that particular gene.

Once you get the expression of that gene like you desired (using PCR), You have to check whether the change in genotype actually correlate to the desired phenotype. For it, you can make a callus from the cells and allow the cells to form the plant. If you really get your trait of interest, you just modified a trait.

Note: In my opinion, there are much better and efficient techniques than miRNA to modify traits. Also miRNAs are pretty new and are not extensively studied. Also, these kind of techniques are tiresome and need a lot of trial and error. Again to industrialize such things, your whole life may pass and you may get nothing at the end.