how to use redirect /fix previous permalink?

this is continuation of
https://www.ads-software.com/support/topic/multisite-in-subfolder?replies=5#post-2214914

I think it’s already become different problem. You said

RewriteRule ^/([0-9]{4})/([0-9]{2})/([0-9]{2})/(.*)$ https://guide4games.com/blog/$1/$2/$3/$4 [L,R=301]

if my domain is
guideforgames.com/2011/11/01/postname

but my domain is
guideforgames.com/postname

so will the rewrite rule become:
RewriteRule ^/([0-9]{4})/([0-9]{2})/([0-9]{2})/(.*)$ https://guide4games.com/blog/$1 [L,R=301]

??
where Should I put the code?? In .htaccesss file?

After doing this and something happen, can it be restored by using a backup (sorry if this question really dumb, because I am afraid that .htacces or any important process didn’t got back up).

Basically I am only need to :
1. define false so I can install in subfolder
2. change my htaccess so my previous post (permalink)can still be accessed, right??
3. just add subfolder site and I’m ready to go, right??

thank you Ipstenu for your answer last time