Most travellers can now get a single or double-entry tourist e-visa for India online for up to 60 days, if arriving by air. If entering India overland or if you need a physical visa for some other reason, you must apply online and then come to this service centre, at the State Bank of India to the right of the Indian embassy, not at the embassy itself.
To make an application you will need a printed copy of the completed online visa form (https://indianvisaonline.gov.in), your passport, a copy of your passport info pages, a copy of your Nepalese visa and, if you've visited India before on the same passport, a copy of that visa. You will also need two 51mmx51mm passport photos (this is larger than a standard passport photo but most passport photo places in Kathmandu know about Indian visa regulations). You will also need the visa fee. Visas take around week for approval; at this point you will leave your passport with the embassy. The following working day you can collect your passport between 3pm and 5pm – hopefully with a shiny, new Indian tourist visa in it.
Visa fees for a six month tourist visa vary depending on nationality but for most nationalities it's Rs 11,000. UK passport holders pay a whooping Rs 17,000.
Transit visas (Rs 2300 for most nationalities) are issued the same day, but start from the date of issue and are non-extendable.