If you have access to a laser cutter, you might be able to get it done locally (assuming you can source the leather). It's smelly and will leave slightly darkened edges (I recommend using masking tape all over the side that's being cut, to protect the leather from charring), but it's fast and precise. Alternately, if you have patience, using an X-Acto knife will do the job just fine on thin leather.
I got the applique laser-cut from chevre (goat leather) for these shoes I made, to give you an idea of what's possible: https://www.dropbox.com/s/t8x3w4dwrpz7fdq/2018-01-19%2013.18.51.jpg?dl=0
If you plan to glue them to your existing leather boots, here's some tips:
As Nik0lla said, your question is very broad but I'll do what I can.
So, foam armor is pretty much the default, especially for first timers, because its cheap and easy to work with.
In theory, all you need is a decent box cutter, a sharpener for the box cutter, a lot of foam, and probably contact cement. That should be a 30 dollar investment, maybe less (of you are interested, I could collect some amazon links). For paint, straight acryllics works on foam without any sort of sealing (although, I would still at least platisip or cover the foam in modge podge before painting) I can go over more specifics on painting later.
Which armor set in particular is it? You should probably start with a gauntlet or something small like that. I can give you a bit of direction, if you are interested.
EDIT: I made a list anyways.... This is pretty much the bare minimum to get started.
Box Cuter: Actually, you can use pretty much any box cutter, but this is a pretty nice one on amazon that should last next to forever: https://www.amazon.com/Stainless-Steel-Retractable-Utility-Knife/dp/B01J8Q3J9K/ref=sr_1_2?ie=UTF8&qid=1490829606&sr=8-2-spons&keywords=box+cutter&psc=1
Foam: Cheap craft foam to start, you can also use floor mats but this is cheaper and easier to work with. My first armor was completely made out of this. This can be found cheaper at Walmart or Michael's: https://www.amazon.com/gp/product/B003LAJGEW/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
Sharpener: This is the one I own, I love it. It's small and really not that expensive. https://www.amazon.com/gp/product/B000WALUV6/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1 WHY DO I NEED A SHARPENER? THAT SEEMS A LITTLE ADVANCE. Well, I sharpen my blade after every 2-3 cuts, its the only way to get a clean cut. Foam dulls knives like crazy, so, it's cheaper to do this than to buy a million blades.
Contact Cemenet: Look up how to use it effectively, it's not hard just different than normal glues (Basically, you apply to both surfaces, wait for the glue to dry, then attach them). DAB is the cheapest option, and you can also but it in more bulk if you're interested (Half of this sort of bottle has lasted me my McCree cosplay, so, one small bottle should be fine for you). https://www.amazon.com/107-3-Ounce-Weldwood-Contact-Cement/dp/B000M2SKNU/ref=sr_1_fkmr2_1?ie=UTF8&qid=1490829763&sr=8-1-fkmr2&keywords=dab+contact+cement
None of this is particularly specialized and should be useable elsewhere! It's all available on Amazon, or pretty much any Walmart.
Optional stuff.
Floor mats: They're thicker foam, basically that's it (Can be found cheaper at Harbor Freight, is there is one near you). https://www.amazon.com/BalanceFrom-Puzzle-Exercise-Interlocking-Tiles/dp/B013A4ATCQ/ref=pd_bxgy_200_img_3?_encoding=UTF8&pd_rd_i=B013A4ATCQ&pd_rd_r=C25J7R5CHZWC1ZP4Z622&pd_rd_w=qVEb3&pd_rd_wg=jHRf0&psc=1&refRID=C25J7R5CHZWC1ZP4Z622
Super Glue: I like Gorilla brand. Some things are just easier to super glue: https://www.amazon.com/Gorilla-Super-Glue-15-g/dp/B001IY82FM/ref=sr_1_1?ie=UTF8&qid=1490830620&sr=8-1&keywords=gorilla+super+glue
Blue Masking Tape: for holding stuff together before you glue, the cheap stuff is fine. https://www.amazon.com/3M-Scotch-Blue-Safe-Release-Multi-Surfaces-Painters/dp/B0062TNT1K/ref=sr_1_3?ie=UTF8&qid=1490830678&sr=8-3&keywords=Masking+Tape+blue
Kwik Seal: For covering seams in foam, is it can be smoothed down with water while still wet (No sanding needed! Unless you want to) https://www.amazon.com/18001-Kwik-Caulk-5-5-Ounce-White/dp/B00062W6MM/ref=sr_1_1?ie=UTF8&qid=1490830733&sr=8-1&keywords=kwik+seal
Rotary Tool (Not Amazon link) Even this 10 dollar one is better than nothing. It's... not amazing but it won't catch on fire while you use it and can generally get the job done: http://www.harborfreight.com/power-tools/rotary-tools/80-piece-rotary-tool-kit-97626.html Obviously, I nicer one would be better, this is just the option for those of use who are cheap. Source: Am cheap.
Heat/Embossing gun: They're nearly the same thing. Seriously, embossing guns just don't get quite as hot, but it's fine on foam, and are SO MUCH CHEAPER. https://www.amazon.com/Darice-Embossing-Multi-Purpose-Heat-Tool/dp/B0013LAYUQ/ref=sr_1_3?ie=UTF8&qid=1490830951&sr=8-3&keywords=embossing+gun
Newsprint: Literal newspaper is the cheapest option and works fine, but one of these large drawing pads is what i use because it's easier to see what I'm doing (I also have like 3 free ones because reasons) You use this to make the pattern of what your doing. Basically, make it with paper and tape first so you don't waste foam, trace what you made onto foam and cut it out then. https://www.amazon.com/Canson-Biggie-Newsprint-Pad-Inches/dp/B0025TX394/ref=sr_1_3?ie=UTF8&qid=1490831046&sr=8-3&keywords=newsprint
Sharpies and Metallic Sharpies: Not bothering with a link because you probably know what these are. Use them to mark the foam (Metallic works best on dark colored foam.) Plus, when you get to the painting stage, metallic sharpies are great for detail work without an airbrush.
As far as painting/sealing goes, this video covers everything you might need to know: https://www.youtube.com/watch?v=ug6Rs06FEpk
That's everything I can think of right now! Will update when I remember more. Feel free to ask questions!
