Need advice installing cutoff valves..

Hello and thanks in advance! I am currently in the process of redesigning a kitchen sink at a home I am temporarily living in. The problem I have encountered is that the plumbing to this home is very old and there are no cutoff valves installed. The only way to cut off the water is to cut off the main supply to the whole house. It's gonna take me a good two days to do the work I need on the sink, so the valves almost seem like a necessity. The water supply to the sink is coming from two 1/2" copper pipes coming through the floor. About 8" to 10" above the floor, the pipes are joined by some sort of adapter to a foot long segment of 3/8" copper pipe. At that point there is yet another type of adapter or coupling that connects it to another 3/8" copper pipe that goes up to the current faucet. There is also a 1/4" copper pipe that is somehow connected to the left pipe just above the floor. The other end of this pipe goes back through the floor and comes up again through a hole behind the refrigerator. I'm very new to plumbing of any sort so I wanted to get some good advice on here before proceeding and messing anything up causing this to be a bigger job. I'm also trying to keep costs as low as possible since I will be moving in a little over a year.

I bought the two valves pictured below at Home Depot for around $16. Are these the correct type to use or is there a better option? The supply lines already attached to the sink that I will be installing are 3/8" plastic.

Thanks again for any advice!

Those valves would work just fine only thing I would change would be to make them 1/4 turn straight stops because they have a ball valve set up but that's not too important. Cut the copper below the brass fitting connected to the 1/2 in copper with some copper cutters and then that valve you bought just compresses on the pipe

Thanks for the quick reply, however I've encountered another problem. I cut off the 1/2" copper pipes clean and went to install the valve and find that the inlet is too big.

The copper pipe measures 1/2" in diameter and the valve I have reads "1/2" norm comp inlet". I can take these valves back and return them but what valve should I get?? Thanks again!