Primer Design With Primer-BLAST
You can manually restrict the design ranges in Primer-BLAST screen.




First of all you should enter the targeted sequence. In this case, it will be NC_000012.12 which corresponds to chromosome 12 that has KRAS gene.
You should determine the position of variant that you interested in that chromosome. The KRAS G12D -or rs121913529- variant is in 25245350. position. You can find the position of specific variant on dbSNP.
For Sanger sequencing, the recommended size is 600-1000 bp. So, we should select ranges +/- 300-500 bp apart from the variant position.
For the left primer we choose From:25244850 To:25245050; for the right primer we choose From:25245650 To:25245850. And then submit.
Here are the primer-sets that you are looking for. :)