era 300 first use

I’m not sure which hole in what speaker you’re referring too. But, the information is given in the product guide for the speaker In question. 
 

What do you plan to do with the One SL’s? If they’re going as a stereo pair in another room, just rename them. 

Are your SLs set up for surround? If not, it doesn't matter, you can just move them to another room if you want. If they are to be sold, they should be removed and reset.

How do you set up your Era300s? as stereo or surround.
On your Era 300 there are 2 inputs as also stated in the User Guide, one is for power the other is a USB C input, you should not use USB C unless you want to use one of these options:

The USB-C port is designed for line-in and Ethernet connections using the Sonos Line-In Adapter or Combo Adapter (sold separately). It's not a power source for the speaker and shouldn't be used to charge other devices.

Ethernet: Use the Sonos Combo Adapter and an Ethernet cable to connect Era 300 directly to your router if you want to hardwire your speaker (helpful if your WiFi is unstable).
Line-in: Use either the Sonos Line-In Adapter or Combo Adapter to connect your turntable, CD player, projector, or other device.

Assuming you want to run everything over wifi?

Thanks guys. I ll probably sell my one sl's or give to my brother. So how do i take them back off my current room please? Thank you so just using the power cable will be fine to connect them them?

Read ​@Peter_254’s reply. He tells you what to do if you’re selling them. Even if you give them away, do the same action. 

If you ONLY have the 2 SLs that are not connected to anything else, just reset them.
Connect power to one Era300 and set it up, then do the same with number 2. Remember to add them to the same room in the App. After that, the App will ask if they should be set up as stereo, here you choose yes.

This is ONLY if you do not want to use them as surround with a soundbar and a Sub if you have one. This is because you cannot connect the Era300 as a stereo pair as surround speakers, they must be added as one unit at a time.
You can use your Era300 with a Sub as a stereo pair.

I have an arc and sub so i will be then hooking the era 300s up to those :-) the one sls that i will sell have been used with my arc and sub but now im just changing them for era 300s, many thanks

Join me in having Sonos Rooms called Spare1 and Spare2, as I didn't like the click-n-pick Room names! :-)

Seriously, just go to settings and select the Room you are changing and edit the Room name to whatever you want.

Many thanks for the replies everyone, really appreciate the helpful advice very much. Ive done a factory reset on the 1sls now and then i will delete and reinstall the app then should be good to setup the era 300's right? Many thanks again all :-)