It's easy to get this map onto your Garmin if you have a GPS that uses a data card (usually a microSD card) - if not, it's also easy to get this map into Mapsource and then get it onto your GPS that way. Here's how:

* Go to OSM Routable Maps.
* Tick the tickbox near the top middle of the page 'enable manual tile selection'.
* The map is divided into 'tiles' - pan and zoom it to your area of interest, you only want a few tiles.
* Click on the tiles you need, to select them (blue colour). If you can't select them, tick that tickbox at the top!
* Selecting an irregular area is fine - or even non-touching tiles is fine - for example, all of India (6 tiles) plus around Heathrow (1 or 2 tiles) works perfectly well.
* Enter your email address in the small form to the left, and click 'build my map', and leave that page, job done.

(Actually, by all means take the time to read the excellent advice on the rest of that page - it's such a fine, useful resource ...)

* Some time later (usually within an hour, but it depends how busy the site is) you will receive an email with web address - usually this is a clickable link, so just click it.
(If an email doesn't arrive, check your spam bucket, it may have gone in there.)
* This email leads to a list of 4 links which are 4 different downloadable versions of the map you requested. You only need to download one of them - depending on how many tiles you selected and how well-mapped that area is, it will be quite a big download anyway.
The file called gmapsupp.img is the one that can be put directly on your GPS data card.
Or if you want, you can download the .exe file instead, which is an installer for Mapsource (on Windows).
The other two files are useful for Mac and Linux users.

* To put gmapsupp.img on your Garmin - if you have a card reader and an adapter for microSD, this is the best way. Remove the card from your GPS (or, much better, use a fresh one) put it in the card reader and connect to the PC. You'll find on it, a folder \Garmin\ (or if you're using a fresh card, you must first make this folder - \Garmin\ ) and inside that folder is the existing map file called gmapsupp.img
Important. Note that, unless you're using a newish GPS model, your Garmin GPS can only read this one map file - that is, the file [card] \ Garmin \ gmapsupp.img
* Back up the existing map file to your PC, then replace it with your new downloaded one, being sure to keep the same filename, and to put it in the folder \Garmin\ - sorry to shout, but this is the single thing that most people manage to get wrong!
* Replace the card in the GPS, making sure it's properly seated. Job done, the new map should Just Work.
Tip - an easy way to spot-check a map for a remote location such as India - load a prepared Track in that area, into the GPS - now find that Track in the menus, open it and select 'Map'.

If you don't have a card reader, then you can connect the GPS and switch it to 'USB Connect' mode, and copy the file that way.

If your Garmin GPS supports maps but doesn't have a card, you can download the .exe file instead. Run it on your Windows PC and it will install the new map into Mapsource (alongside any other maps you already have). Now you can get the map into the GPS.
Actually this method has an advantage - using Mapsource you can mix maps, for example a Garmin UK map plus a OSM India map - all available on the GPS at once.

If you don't have Mapsource or Windows, then you'll need to try the other downloadable files and the various online help pages linked on the main map page. There are lots of ways to skin this cat - what I've described above is just the easiest one.