Set up IAST keyboard to type Sanskrit using Latin characters in Windows 10

English Language (India) pack uses ISO 15919 scheme of transliteration; not IAST. These two schemes are very similar. Go to the following link to compare these two schemes of transliteration.

https://en.wikipedia.org/wiki/Devanagari_transliteration

In short, IAST is the academic standard for Sanskrit transliteration. On the other hand, ISO 15919 scheme can be used for any Indic language transliteration.

Click on the "ENG" at the right corner of the taskbar. Then you will see the "Language Preference" option. Click it. Settings page opens. There click on "add a preferred language". Select "English (India) language pack." Install it. 

Now, from the taskbar, click on Eng. Select Eng (India) keyboard. Now your keyboard is English (India).

Now you can type. 

ā, Ā, ī, Ī, ū, Ū, ḥ, Ḥ, ṁ, Ṁ, ṭ, Ṭ, ḍ, Ḍ, ṇ, Ṇ, ś, Ś, r̥, R̥, l̥, L̥ - for these letters, press right Alt key+the respective letter.

For capital letters, press the Caps Lock key as usual.

ṣ, Ṣ - right Alt key + x, right Alt key + X 

ñ, Ñ - right Alt key + y, right Alt key + Y

ṅ, Ṅ - right Alt key + g, right Alt key + G

Here is the link to the English(India) keyboard layout.

https://docs.microsoft.com/en-us/globalization/keyboards/kbdinen.html 

To follow the IAST transliteration scheme, you may use the Heidelberg Input Solution Keyboard from Keyman Desktop. It is free and open-source. You can find the user guide there. It is easy to use. Here is the link -

https://keyman.com/keyboards/heidelberginputsolution

Another keyboard is Indic Roman Transliteration (SIL) from the Keyman desktop (https://keyman.com/keyboards/sil_indic_roman). You may use this one also. I personally prefer the Heidelberg Input Solution.