As appears from your query that you are giving services from India to your B2B customer situated in Singapore. As far as Installation services are concerned the place of supply is in India and you need to pay GST in India.
We are not sure about Singapore GST but as per the basic structure of GST if you are billing from Singapore then surely GST will be applicable. Better you form a company in India and bill directly from here to your client to avoid dual GST.
In India, there is a registration threshold of INR 2 million for services. However, if you are taking NRTP (Non-resident taxable person registration) then no threshold limit applies.
In India, the GST rate is 18%.
Furthermore, to add up since this transaction is a bit complex in nature better if you take the written advisory from a professional so that your risks are limited. Also, cross-border transactions especially between related parties also are to be valued carefully so that there is no short payment of taxes.
Trust this meets your clarifes,
In case of any concern feel free to revert.