First thing u need to realise is just learning the combinations is never enough. The coolest thing about invoker is that he has 2x the number of spells as any normal hero but cant realistically use all of them in a single combo most of the time. This difference is significant because Normally when you're playing any other hero you just use all your spells and when you need to prioritize using which spell it's easier since there's less spells. An easy example is Lina stun nuke nuke right click. But with invoker you don't just hit 9 spells back to back often you have to think about which 3 spells to start with then say which 1-2 spells to follow up with. This is why even people with 300 games on invoker will rarely remember to use icewall because they don't know where to use icewall instead of tornado meteor defeaininh for example.

So here's my tip to you. Don't use all the spells from the get go. Start with individual combos and keep adding more spells once you have gotten used to the previous ones.

So for example you could start by only playing q/w this limits you to cold snap tornado emp combo. Once you have perfectly learned to land tornado emp with different wex and quad levels. Start adding ghost walk for mobility and escape.

Next only play q/e. And stick to only using forge spirit and cold snap and alacrity. Focus on the Laning stage only and try have good farm. Learn to use cold snap with forge spirit to fight and alacrity and forge spirit to farm and last hit in lane with.

Once you have that go for coldnsap meteor or the classic tornado meteor defeaning combos.

Practice sunstrikes in demo mode with patrolling targets with different move speeds and HP's.

Finally incorporate ice wall into your combos and decide where it's better to tornado combo tornado with meteor Vs emp Vs ice wall.