As far as I can tell, there's no link to metas on the actual SE sites. You have to first click on the "ask a question" link, which for some reason is all the way over to the right despite it being the whole point of the site, and then you have to notice the "Ask it on meta" option over the right, at the bottom of the "How to Ask" box.
If you click on the 'hamburger menu' (upper right corner, see first pic below), you will see a menu (second pic below). The first entry on the menu is your current community; immediately below that is the "meta" for that community. If you select the Meta link, you'll be sent to the Meta, which has its own "Ask Question" button, which works just like the regular site.
Asking on meta of a specific site, has the same procedure as asking on a regular site. Namely you first have to go to a specific site, and when you press on ask question it will automatically bring you to asking the question on the site you were on till now.
To access the Meta site simply press on the stack exchange icon on the top right of the screen, and you'll see an option to visit meta of your site. When in meta, press on ask question like you would do on any other site.
In addition to the hamburger menu the other answers mention, another way of getting to a meta site is opening one of the "Hot Meta Posts" on the right sidebar. (That is how I usually do it, as it requires one less click.)