Sketch the Resonance structure
As can be seen from the periodic table below, oxygen, sulphur, and selenium belong to the same group, and they are all non-metals, so one should expect their chemical properties to be similar.





Having established their chemical similarity, one can transform the two ions in question into more familiar forms.

In thioformate anion ($\ce{HCSO-}$), we are not too familiar with sulphur $\ce{S}$, so let us transform it into the more familiar oxygen $\ce{O}$ whose chemical properties are similar. Then, it becomes formate ion ($\ce{HCOO-}$), whose resonance structure is well-known:



Just transform one oxygen back to sulphur and we are done:





In selenocyanate anion ($\ce{[SeCN]-}$), we are not too familiar with selenium $\ce{Se}$, so let us transform it into the more familiar oxygen $\ce{O}$ whose chemical properties are similar. Then, it becomes cyanate ion ($\ce{[OCN]-}$), whose resonance structure is well-known:



Just transform the oxygen back to selenium and we are done:





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The first image is copyrighted by Michael Dayah.
The second image is from Wikipedia.
The third image is a derivative work of the second image.
The fourth image is taken by HGTCChem.
The fifth image is a derivative work of the fourth image.