The red  leash is used for the front of the feet.

The blue one goes behind the 'ankle'/feet.

The large collar (a) just goes around your feet like you would do with regular
sandals.

The small black collar , I use it to go around my ankle, so it keeps the blue
one secured to my feet for extreme conditions ;).

The blue (b) and (c) are optional, I have another pair of sandals without
them, and it works fine with most type of feet
(just that with my lion the feet were 'round' and I was always loosing the
sandals so I added these on the 2nd pair)

Here you see how it fits on the feet :)

Building:
+ To start, cut 2 pieces out of the foam camping mattress ,
 the size you would like your sandals to be.
you can give them the shape you like, 
either a basic oval shape or anything you like
Make them fit as well as you can to the shape of your fursuit feet.
Keep it mind that it might move slightly when walking, 
and add some extra in case you have long fur on the sides 
of your feet that might be touching the ground. 
(But keep in ming you will still have to walk with them ;) )

The ones I made were originally made for my plushie lion, 
but I've been using them fine with both timdurus too :)

+ take one of the piece  and cut slits with a cutter,
the leashes / collars(L/C) will go through these.
you can either get the  L/C to go all the way under 
that foam piece as shown in the photo

or you can also make it go partially under. 
I tried both and didn't have any problem with the partial , so I guess you
could go for that so you can save some L/C length. :)

As shown  in purple, you should reinforce the
openings/slits on the inside ( that will go in between the 2 layers of foam)
You can use strong plastic, or a piece of leather for example.
(I only pictured it for the (a) but you should do it for (b) too )
Cut a slit through that material and glue it on the under of the current foam
piece.
Then for each L/C get them through the appropriated slits and glue them under
the current piece of foam
You might want to try how it fits on your fursuits feets, by pinning them
first instead of glueing just to make sure.

For the front red leash, it has to be fitting very tight on your fursuit feet. 
For example if your feet are made of foam, it should compress the foam enough to keep the
sandals in place. 
As you can see on the picture, and yes it'll go through ;)
Depending on the shape of your feet, 
or if you prefer not to compress too much to keep the sandal in place
you can also add another piece of leash, 
starting perpendicular from the middle point of this one. 
(just like real sandals usually have) 


Once the glue is dry enough, 
get the 2nd piece of foam, 
and simply glue it slowly to the bottom of the first piece. 

and that's it for the basic type of sandals.