There is already a (partially) similar paper, look at its methods to draw some ideas. If you follow the paper methodology, upgrade the tools used (hisat - stringtie - ballgown onstead of tophat - cufflinks). There is a Cannabis genome, unfortunately, it seems it is not annotated.
First of all: What kind of data do you have? Is it short read data? Is it long reads (e.g. PacBio SMRT/Nanopore?).
If you have short read data, try SOAPDenovo for assembly. In case you have long read data, I had pretty good experiences with "Canu".
For Variant calling I assume that GATK should work - but didn't use it on plant data yet.
DESEQ2 should work fine, just make sure to use a proper reference genome when mapping reads and then annotate properly before doing the expression analysis.
There is already a (partially) similar paper, look at its methods to draw some ideas. If you follow the paper methodology, upgrade the tools used (hisat - stringtie - ballgown onstead of tophat - cufflinks). There is a Cannabis genome, unfortunately, it seems it is not annotated.