After the opening mission, you should have some Meld.
Research "Meld Recombination" to make the Cybernetics Lab and Genetics Lab available to you.
Next you need to build the Genetics Lab. Once the construction is complete, under the Research tab will be Visit the "Gene Lab".
The cheapest gene mods available at the beginning of the game are the eye mods: either one costs $15 and 5 Meld. Choose any solider you like and after three days, the trophy will unlock.
Alternatively, you can to go a bit further ahead if you have the Thin Man Autopsy available. That way, you will unlock the Muscle Fiber Density leg mod for $60 and 25 Meld, which is necessary for a separate trophy.