There is one all-absolute way.
Disconnect all your hard drives and usb drives, printer, and anything at all that has any kind of memory at all.
Download on a known safe connection the .ISO of the Linux distribution of your choosing, such as Ubuntu or Linux Mint.
Write that ISO, (ISO's are optical disc images such as DVD images or CD images) to a DVD, won't be small enough for CD's anymore, and then boot up with that disc that you wrote your chosen Linux ISO too.
Ensure before you boot that all hard drives are disconnected, and all usb drives are disconnected, and any device that has any kind of storage memory is disconnected. That means disconnect printers, and use simple cheap keyboards and mice that don't have those advanced settings storage. You must disconnect drives and printers and other devices with memory physically if you do not know how to make a custom Linux ISO that can't mount that device.
Don't worry about RAM memory. RAM doesn't store memory when power is not applied. It irrevocably disappears within seconds of losing power.
Then with your simple no memory keyboard and no memory mouse, and no other devices with memory, boot up with your Linux DVD.
Ubuntu and Linux Mint both have many internet-necessary programs such as flash player already installed on their provided ISO's.
If your Linux distribution does not, like maybe Fedora, then you'd have to make a custom ISO, and include those items. Good luck with that. It's not easy, especially if you're not used to using Linux.
Now, with all devices that store any kind of memory long-term physically disconnected, and using a keyboard and mouse that doesn't have any kind of memory for settings or something, and no USB storage devices connected, there will be absolutely no chance of being infected with malware no matter what you do.
Even if you installed it yourself, without anywhere to jump too, it will only be present for as long as you run the session. After you shut down, it will go away along with all the data written to your RAM during the boot up process.
The only way, and this is a big IF, would be if somehow the program fooled the machine into keeping power as though the OS was still going, which kept power to the RAM that the livecd installed your linux distribution too. However, you can avoid that potential, if it's even possible, by simply unplugging your machine for a few minutes before you reconnect your storage devices and such.
There is absolutely no way to get malware onto a machine if you have nothing for it to jump too, and your OS is limited to only using RAM to run, which is the case when you run a Linux livecd.
It actually installs the linux OS to your RAM. A lot of people run linux this way to avoid malware and to ensure that their OS environment is completely secure from potentially hidden malware.
It's a great way to access you bank because when there is no storage device mounted, the malware would have to be able to mount the device, and then write to it, and that's hard to do without the user knowing it when running a livecd. Although, it's getting easier because of how seamlessly and effortlessly we can mount storage that is on the machine.
That's why the absolute only way to avoid malware is to disconnect anything with long-term storage of any kind, including printers and keyboards and mice, and boot from a livecd, ie. the DVD ISO you downloaded.
Of course if the NSA got your RAM chip provider to secretly install a permanent storage hidden away so that only the NSA could access it, and some hacker discovers that and exploits it, then even that won't be the absolute best way.
That's probably why Bruce Scheneir recommends the use of a machine that you can be sure has never ever ever been connected to the internet . . . ever. That means you'd have to build it yourself, basically, with brand new parts that you can guarantee have never been exposed to the internet. But then, that's only feasible for someone who actually does work on the machine that they don't need the internet for. He said he uses his to encrypt important data.