These security settings are not easy to get right at first.
My usual advice is to first try and implement that tutorial exactly as it is, with those exact user names and role names, until the point where you get it working.
Only then, start changing things around to fit your Company.
A couple of extra tips I can give you:
People often neglect to assign the security groups to the records. Any security only comes in effect once records (Accounts, Contacts, etc) are assigned to one or more security groups.
There is a nice feature to check the user side of the permissions. This is, let’s say, half of the problem you need to solve (the other half is the side of the records being accessed). Go in “Roles Management” and then look for an option on the left-hand menu called “List Roles by User”. There you check which Roles each user is getting (the most permissive from all the groups they’re in).