Carpeting Stairs in Four "Easy" Steps - Pun Intended

Carpeting stairs is a lot harder than it looks! But to be honest, I haven't seen it done, uhh, ever, actually. We've never done it before, and we relied heavily a lot on YouTube videos which were very helpful. So here are the "four" steps (it's actually quite a few more, but it's four main steps).
Our stairs had already been stripped of this lovely runner (one of the first things we did after moving in, for hopefully obvious reasons):

so we didn't have to remove anything when we were ready to carpet. Just for the record, by the way, I didn't really want to carpet the stairs, especially after seeing so many cool ideas on Pinterest, like this and this.
But for resale value, carpeting made the most sense. That, and the fact that these particular stairs are basically at a 45 degree angle, so it's a safety thing as well. So back to the steps. Step one is to clear any existing carpet and gunk and dirt from the stairs:

So clean! And so white! Let's change that, shall we?
Step #2 is to install your tackless strips (aka tack strips). These are pieces of wood that have super sharp metal points coming out at an angle to help hold the carpet down. Measure your stairs' width and cut each strip to that size. Then you want to install one 3/4" from the bottom of the riser (the vertical part of the stairs) and one on the tread (the part you step on), touching the riser. See picture:

Also, make sure the points on the tack strips are pointing DOWN for the one on the riser and UP for the one on the tread.
Also make sure when you're hammering them in that you don't hammer too hard or this might happen:

Whoopsies. Good thing we decided to put carpet on these babies!
Step #3. Now you want to cut your pieces of under padding (if you decide to use this). If you have an old house, make sure to measure each stair individually! The reason we decided to carpet each stair individually is because (after researching on Google) it lasts longer. If you use one piece of carpet for the stairs (the "waterfall" method), eventually the carpet will start to come off, and if each stair has their own piece you can replace it easier. It's more time consuming doing it this way, but is better in the long run. Anyways, back to step three. So you want to measure the width of the stair, and the "length" (the tread and riser - make sure to measure around the 'bullnose' too if your stairs have that). But don't include the tack strips in your measurements. Basically, measure from one tack strip to the next. Cut your piece out, and get your handy husband and his handy pneumatic staplegun to install each piece:

This picture shows some utility knives, and I started out using them, but I found that just plain kitchen scissors cut it so much easier and faster.

If it looks a little wonky it's because it is. Under padding is made of lots of tiny pieces of foam, and staples don't like to hold it that well. But it'll work.
We took a step back and looked at the stairs after this step was done, and we both said we should just leave it like this. It looks kinda cool! But then we'd have all this carpet and wouldn't know what to do with it, so we just decided to keep going.
Step #4 is to cut and install the carpet. Same as with the under padding, measure each stair individually. This time, include the tack strips in your measurements. I recommend doing a test piece first, in case your measurements are off (like ours were):

Starting at the bottom of the riser, place the piece of carpet on the tack strip and smooth your hand along up to the bullnose. Staple the snot out of it at that joint, and then continue smoothing your hand over the tread and push the carpet into the little crevice created by the two tack strips. It should hold just like that, but we used carpet with a kanga backing, and found that the tack strips didn't hold it too well so we used staples too.
Go all. the. way. up. and you're done! Don't they look so furry?!

Install the trim, the railing, and vacuum all the little pieces of carpet fibers away and you have a nice cozy set of stairs that don't squeak anymore! In just four steps!