Lots of good advice here...
As far as what I do - I have things set up a bit funny* for my manually focusing a tele lens... but the gist of it is to turn the focus ring until the focus looks good (sometimes having to rock back and forwards on the focus and find the middle ground...) and take a photo.
If you're interested in catch-in-focus, take a look at this article here, it's got all the info you need:
Catch-in-Focus Tutorial - Articles and Tips | PentaxForums.com
I find that it can be quite useful for birds in flight...
Sometimes, with more tricky subjects, the answer is to simply use burst mode and take several images, deleting the out-of-focus ones...
Since you're interested in landscape photography, you have a bit more time when manually focusing there. You can set up on a tripod, open live-view, and zoom in. Focus peaking is also a choice - I've always disliked using it, however I'm starting to use it more for flowers shots and landscapes and finding a bit of success...
Overall, the real answer is practice. It takes time to become proficient, I've had that A 400 (used in the shots above) for two years now and only recently have I been able to focus quickly and more accurately.
*set up with catch-in-focus on the shutter button, and cancel AF on the back button, so I can take a shot I deem in focus that the camera doesn't... except for this one user mode, in other shooting I have back button only AF as I like focus and shutter to be two completely different functions... I also have all the beeps and things turned off, so I ignore/not notice focus confirmation half the time...
Last edited by bertwert; 05-19-2020 at 06:14 PM.