You can do this from your main account, but the problem is that cards you list on the rental market will still be playable. So if you list a card for rent that is actually the highest level of a card in your account, you could select it for battling by accident, which triggers cooldown.

So, personally, I have all cards that I rent out in a separate account.

On Peakmonsters, you can check the prices for the cards that are being rented out. Let's say you want to put up a Water Elemental level 3 up for rent, then you can either go to the market place, and check for how much other people rent out the same card at the same level. Then, you pick a price about in the middle - generally below 5%.

Peakmonsters now also has a new tool: when you're listing cards for rent, it will show you a list of other rentals for that card and the prices, so you can pick a price similar.

It's pretty self-explanatory once you dive in.

This video was made a couple of months ago, before the new tools on peakmonsters were released, but it might give you a good idea of how and where to start
https://steempeak.com/splinterlands/@peakmonsters/2videos-to-explain-renting