New GM, few questions about abilities and attributes (without premade character sheet)

Hey, so me and my group wanted to switch to roll 20 for practical reasons and I (GM) have naturally been assigned the task to understand how it works. We play with a homebrew set of rules so finding a sheet that worked fine for us was impossible. That's when I discovered that there is a cool basic sheet thingy for when you don't select any. Thing is, I can't really seem to make it work and documentation on it seems very poor. The main question I have is about maths in abilities and attributes, how do you add two attributes into another inside an attribute or ability ? It seems to work fine if I roll a dice and add an attribute to it but when I try to add them without rolling the dice it just writes x+y in chat

They need to be part of a roll, which doesn't mean there are dice. You can use inline [[@{name|attr1} + @{name|attr2}]] If you're putting this in chat, you must reference the character name.  If it is in a character ability, you can omit that.  Abilities are just a collection of chat commands (aside from the reference shorthand), so if you can do it in chat, you can do it in an ability.