Below is the microsoft link to install technical preview and list of supported phones.
The steps roughly are:
1. You have to install windows insider app
2. Select, whether you want slow/fast ring updates. The difference is with slow, you have stable builds, and fast means like hot cooked meals, as soon as the developer gives it a go.
3. If you are not app developer and enthusiast like me, better to select slow update
4. After you selected, the insider app, asks you for reboot. Allow it
5. Once rebooted you can goto phone update in settings and when you check for updates, it will show you a windows 10 update. Go ahead and click update. If space is less, it will prompt you to free up space in phone storage. Do this. I even removed whatsapp, so that atleast 1GB of free space is visible after it prompted me. While you are freeing the storage, update continues in background.
6. Make sure you have enough charge while it updates, if not leave it for charging during update
7. After installing it asks for reboot. Allow it
8. Leave the phone after reboot for an hour. It takes pretty long time
The real problem starts after it updates, if you have an SD-card. It asks, whether you want to fix SD-card issues. If you select "NO", your apps will be unavailable that you installed on SD-card, along with any files on it. If you select "YES", it says 'scanning' but the end result is same as if you would have selected "NO".
The solution I found for this SD-card problem is multiple restart-multiple scan. Select "YES", immediately after reboot and let it fix SD-card issues. Now after 10-15 minutes, again reboot the phone, and again when the prompt comes, select "YES". Now doing this for 5-6 times, you realise that after reboot, your phone no longer prompts for SD-card issues. Leave it that way, and all the apps installed on SD-card become available. Mine is a 16GB Sandisk micro SD card.
Below are some of the screenshots on my Lumia 730, after installation.
The build I installed is 10586.29. Hope it helps. Also, be careful while your phone updates and after reboot at 8th step above and don't press anything. Any other queries comment, will try to reply as soon as I see your comments.