hi!! absolutely no worries, i don't mind answering questions <33 below i've linked some programs/tutorials that i personally think are useful and have helped me a lot, especially when i was first starting out.
programs
the first program i reccommend is 4k video downloader. this is what i use to download the videos i want to gif. while there are other methods you can use, like using your laptop to screen record specific clips, or screen capping (which is when you use a sequence of images to make gifs rather than clips of videos - i'm not super knowledgeable on this), but this is the method i prefer the most.
this next program is optional, but if you want high quality gifs, i highly recommend installing vapoursynth. vapoursynth allows you to clip specific timestamps from videos, resize them to fit tumblr's dimensions, and sharpen them. it can be complicated to use at first, but i promise it's easy once you're used to it!! again, this program is optional because you can technically do all of this in the programs listed below, but i use it because it helps with the quality of my own gifs. here's vapoursynth for windows, and vapoursynth for macos (and both of these links are complete with guides that walk you through the installation process).
adobe photoshop is the most common program gifmakers use to make gifs. you can do basically everything in it - clipping, resizing, sharpening, coloring, etc. but if you can't afford to pay for it (like me), you can pirate it :D this document has a list of links you can use to pirate photoshop. make sure to download the right one (windows vs macos) and follow all the directions listed so that it goes smoothly.
if you feel overwhelmed by photoshop and don't want to download a program that takes up so much space on your laptop, you can also try photopea!! i actually did use photopea at first when i started out making gifs. it's basically a web browser version of photoshop, and while it doesn't have all the features, it's still good enough for making gifs.
tutorials
there are so many different methods for making gifs, and if i went over them all, it would take forever lmao. so i'm just going to link some of my favorite tutorials, and you can look through them and decide what method is best for you. the ** indicates the tutorials that are the most similar to the methods i personally use.
i hope this is helpful!! feel free to reach out if you have any questions regarding any of this information <33