I haven't seen many thief boot tutorials out there that really SHOW you what you're doing. I learned that making these boots is harder than it looks. So here is a tutorial to hopefully help you ease into it better than I did :)

This is the link to the photo collage, and it is numbered by stages so you can follow along: GoddessandtheMuse

First of all. You have a few easy options if you want to just buy some boots and modify! If you're going canon, Rikku's boots are kind of leathery looking with a gloss, and a SLIGHT heel, with a tongue in the middle. Reference: Boots                               

 Gardening and Rain boots have enough qualities in common to easily be modified, but I've found the best choice is Cowboy boots because they already have the white diamond shape built in, making the painting far easier. I found mine for $4 at the flea market, so I urge you to try thrifting before throwing down $20+ on new ones. Now on to the steps! (the photos are numbered in reference to the steps.)

1. Cut the top half of your boots OFF. Be sure to check the reference photo, as you don't want the cut to be too far down. Better longer than short, so cut little by little until you're satisfied. Once the top is cut, you need to shape the tongue! It took me about 4-5 minutes to do this part, because I did NOT want to chop it too far down. Just cut the top part slightly shorter than the rest of the boot. Then, if you wish, cut down each side of the tongue, but do NOT cut it off. Just cut down the sides and stop a little above the white diamond shape. It won't stick the boot, but we'll fix it later.
2. Spraypainting: Start with blue. You'll need to cover the areas that will be white, and there are many methods for this. I covered mine with electrical tape and a plastic wal-mart bag! (as you can see in the photo). Alternatively, you can get some cardstock, foam, or foil and cut a shape in the middle, and you can center the parts you want to spray into the opening, while hiding the non-painting parts behind it. It actually works quite well, but be careful of drips! Now, spray the boot (according to your ref photo) blue. 
3. Once the paint is dry, you can remove your coverings if you used any. The parts that will be white are now already lined off. 
4. VERY IMPORTANT. In my photocollage, I used the electric tape/plastic bag method again to spray the white. DO NOT DO THIS. The tape will rip off some of the blue spraypaint. Instead just use the cut-out method I talked about above, or another no-tape solution to be sure you don't spray the white on the blue. 
5. Before spraypainting the white, you need to decide how you want to cover the top of the boot. Since you cut it off, it will fray and look kind of odd and ugly. There are many options for this too. I've seen people use fabric and piping. I initially planned to use thick wood glue and sand it down to shape. But I ended up going with puffy paint! You can find puff paint in the fabrics section of Wal-mart or T-shirts section of Hobby Lobby. (I tried to take a decent photo.) Anyway, you'll want to line the TOP of the boot with whatever you choose, and optionally, the side too that will be white. I only lined the top. Now, that tongue is still wobbling all about, so it's time to stick it to the boot. You can use superglue or hotglue to stick it to the INSIDE of the boot, do not stick it side to side of the initial cut. However, I lined the sides of the cuts with puff paint and lightly pushed the tongue against it, filling in the gaps on each side. This stuck my tongue to the boot easily, but it CAN pull off if you'll be doing any running or jumping. The puff paint will take about 4-6 hours to completely dry. I tend to leave it for an entire 12 hours before messing with it.
6. Once that's done, you can spraypaint the whites! If you look at my photo you can see that the blue and white bled onto each other in some spots. You'll probably have to get some acrylic paint and hand paint some parts, because it was VERY hard to get a straight line with the spraypaint and not bleed it. Luckily the white acrylic blends well with the white spraypaint. (My photo pictures the ribbon already in, but put in the eyelets AFTER painting!) 
7. Finishing touches: Eyelets are optional, but they are canon to the character design. Some people use a line of 4, and some use a line of 5, so whatever works for you! Get some regular size (large at wal-mart) eyelets and an eyelet tool, and also a hammer. Check the directions for use of eyelets! This part was not easy at all. I went through 2 packs of eyelets before they all stayed hammered together, the leather on the boot can be a huge hassle, but the end result looks very fancy! For the lower eyelets you'll have to put the little black piece inside the boot, and carefully push the boot and it to the side enough that you can hammer it down! Once you have all your eyelets hammered in, you can tie in your ribbon. I did the corset stringing, where you stick each end into the bottom eyelets and then pull them through the opposite sides before you start crisscrossing. Then just tie up the top!