I have detailed out the steps in easy to follow ways.
STEP 1: Backup your current Windows and files.
This is fairly straightforward. Most laptops and PCs come with a backup tool. For my Samsung Laptop, the tool is called Recovery.
STEP 2: Download the Windows 10 Technical Preview
There is a reason I didn't make this the step 1. The reality is that doing a full system backup will take as much or more time than downloading the Windows 10 Technical Preview. So once you've initiated the full system backup, you can download the Windows 10 Technical Preview by following this link. If your PC is 64 bits then download the 64 bits version, otherwise download the 32 bits version.
STEP 3: Prepare A Partition To Have the Windows 10 Installed
You will need to have a separate partition to install the Windows 10 Technical Preview. If you don't have any free/empty partition to use, you can shrink one of your existing partitions to create a new one from it. It's very safe and it won't damage the existing partition. You will only be taking some of its free space and creating a new partition from it.
Open My Documents, and on the left pane, right click on Computer or This PC (whichever you see). Then click on Manage.
In the Computer Management window that comes up, on the left pane, click on Storage, then Disk Management.
All your existing partitions will show in the right panes. Now, go to the lower right pane and rightclick on the existing partition you want to shrink.
Specify the size of the new partition you want to create. (In the example above, I specified 100GB). Then click shrink.
Once the shrinking is done, you will see a new partition called Unallocated. That's the one we will use for the Windows 10 Technical Preview installation.
STEP 4: Make a Bootable USB Flash Drive.
We are now halfway done. You will need to make a bootable USB flash drive from the Windows 10 Technical Preview you downloaded. Get a USB flash drive that is 4GB or above. Plug it into your PC. Back it up if you have valuable files in it as it will be formatted.
To make the bootable USB Flash drive, download Rufus.
Once you've downloaded it, run it and you will get a window like the one below. Ensure the Flash drive to be used is the one showing under Device.
See what the arrow in the screenshot above is pointing to? You will click on it to load the Windows 10 Technical Preview you downloaded.
Next is for you to verify the partitioning scheme used on your PC. If it is MBR or GPT. It's easy to find out. Remember the computer management window where you did the shrinking? Go back there and rightclick on the Disk name in the lower right pane. Click on properties.
You will see a Window pop-up like the one below. Under Volumes you'll see the Partition Style. The one below is MBR.
And this one is GPT.
Note which yours is.
Now go back to Rufus and select the Partitioning scheme that corresponds to yours. If yours is MBR, go with the first MBR option.
Click on Start.
And close once it's done.
And that's all for this step. We have made a Bootable USB flash drive.
STEP 5: Reboot And Install
This is the final step.
Reboot your PC and press F2 or F10 (depending on your PC type, but one of the two will almost always work) to set your PC to boot from USB drive. Then follow the easy steps you will be shown to install the Windows 10 Technical Preview.
Then you will be taken to where you'll specify the partition to use. Choose the Unallocated partition you created in step 3. The installation will begin. When you get to a page prompting you to choose settings, go with Use express settings.
And you are set.
And that's all!