How to finish corners on baseboards?

take some scraps of wood to practice on..in stead of a "butt" joint, that's the type in your pictures do a beveled edge cut the wood trim with a 45 degree slant to it. it should make a point when both pieces are matched up.im trying to get the I pad to sync with the desk top....if I get it synced, I'll show you what I mean, actually- Its the same way most picture frames do the corners.

You need to miter your corners or get corner pieces like this:

On the top you can use a block.

https://www.lowes.com/pd/1-125-in-x-6-5-in-Interior-Primed-MDF-Inside-Corner-Baseboard-Moulding-Block/1000301825#

I would have invested in a mitre box. Still fixable with other trim. Bottom photo I would probably replace the trim on the right side and either go with the corner block or mitre the corners.

https://www.lowes.com/pl/Baseboard-moulding-blocks-Baseboard-moulding-blocks-Moulding-millwork/4294518244

https://www.bobvila.com/articles/how-to-install-baseboard/

https://www.lowes.com/c/Moulding-millwork

I recommend using pre-made corner pieces. If you want to do your own corners, you will need a miter box and saw. Cut wood on a 90 degree angle and the edges should meet. Practice first to get the corners you desire. When we put our baseboards down, we tried the miter box method until giving up. Pre-made was so much easier. It's ok if the corners are higher than the baseboards. It add character. I hope you find this info helpful.

That is not a corner block. It's a plinthe block. Usually used at the bottom of wide door casing or the bottom of the pilasters of a fireplace mantle. You need to either miter the corners or use actual corner blocks like Ann suggests. In you case since the baseboard is already cut get the premade corners.

You van cut 45 grade or the easy way cut a square block as the high of the panel in this case 4x4 cm.