If you are simply talking about the act of standing on the shoulders of giants, the following phrase with an infinitive and super with the accusative of place should work just fine:
stāre super [h]umerōs gigantium
to stand / [the act of] standing upon the shoulders of giants
If you are talking about a person or thing standing on the shoulders of giants, you should opt for a participle instead (again using super with the accusative of place). For the sake of simplicity, I will assume that the person or thing is simply a vir, virī, m., but you can obviously use whatever suits you:
vir stāns super [h]umerōs gigantium
a man standing upon the shoulders of giants
The case and number of vir and stans should be modified accordingly based on their function in the context(s) in which they are used.
Note that the noun humerus, humerī, m. is simply an alternate spelling of umerus, umerī, m., hence my annotations.