So you wanna install Open Fortress...

If you were linked to this, it was because you're asking about installing Open Fortress, or you had some problem with one of the solutions below. Please read this entire page before continuing, it may have the answers you're looking for.

If you posted in general, please use the troubleshooting channels in the future. It's a rule in the server that you must post support issues there. It's also not helpful to people trying to discuss the game, there has been a huge influx of people asking about problems in the wrong channel.

There are a few options for you in terms of installing Open Fortress. A brief overview and list of known problems are listed alongside them.

OFToast

OFToast is an installer for Open Fortress. It's the most popular way to install Open Fortress, but isn't supported on Windows 7.

Install Source SDK Base 2013 Multiplayer and Team Fortress 2. You can install the prior by going to your library, clicking the drop-down box labelled "Games" and ticking "Tools." Now search for "Source SDK" and it should show up. Ensure it is the correct version.
Download the launcher. You can find it in #install-game-md.
Launch it. Your antivirus may mark it as a virus, add an exception or temporarily disable it.
If it isn't already automatically selected for you, browse for your sourcemods folder. Your sourcemods folder is only on your primary drive! On Windows it's typically in C:\Program Files (x86)\Steam\steamapps\sourcemods. On Linux it's usually ~/.steam/steamapps/sourcemods or ~/.local/share/Steam/steamapps/sourcemods.
Hit install and let it finish.
Fully restart Steam by clicking Steam -> Exit at the top left corner and opening it again.

Known Problems:

If you get an error about invalid URLs, HTTP, timeouts, connections, or the server, it's because the primary Open Fortress servers are down because of heavy traffic. You can switch to https://toast-eu-1.openfortress.fun/toast/ if the main one is down. Though, this may also be down.
If you've upgraded your game but you're getting an error about your game being out of date when joining a server it can mean that either you're using a broken install, or the server is outdated. Both of these are possible. To fix a broken install, use the repair version of OFToast once and try again. You can also use murse's repair function as described in the first known problem in the murse section. If it didn't work it means the server is out of date.
If your game isn't launching, or there's missing textures, run the repair vesion of OFToast once and try again. You can also use murse's repair function as described in the first known problem in the murse section.
If your game isn't showing up in your Steam library, ensure Steam is fully restarted by clicking Steam -> Exit at the top left corner and opening it again. If that doesn't work ensure Source SDK Base 2013 Multiplayer and Team Fortress 2 is installed.
OFToast will give you errors on Windows 7 as it's unsupported. You'll either have to upgrade your system (reccomended) or use another method.

murse

murse is a command-line program for installing Open Fortress. It's compatible with OFToast and is considered more reliable and stable. It's also compatible with Windows 7. There's a video guide here for installing Open Fortress with it on Windows, and one for fixing corrupted installs here which is more noobie-friendly than the below guide (which assumes you have some knowledge of the command line.)

Install Source SDK Base 2013 Multiplayer and Team Fortress 2. You can install the prior by going to your library, clicking the drop-down box labelled "Games" and ticking "Tools." Now search for "Source SDK" and it should show up. Ensure it is the correct version.
Download the executable from here. Install this to your PATH or move it your sourcemods folder and invoke it there.
Run murse upgrade $SOURCEMOD_DIR/open_fortress. Your sourcemods folder is only on your primary drive! On Windows it’s typically in C:\Program Files (x86)\Steam\steamapps\sourcemods. On Linux it’s usually in ~/.steam/steamapps/sourcemods or ~/.local/share/Steam/steamapps/sourcemods. The program should start logging the files being added.
Fully restart Steam by clicking Steam -> Exit at the top left corner and opening it again.

Known Problems:

There was a bug in older versions of murse (labelled 0.0.0 with --version) that caused it to corrupt files during downloads. It's very likely you have corrupt files if you installed your game using that version. Luckily, v0.0.1 introduced a fix as well as a command for fixing corrupted installs without doing a full download. Simply run murse verify -r [game directory] to fix your install. This also works for OFToast installs.
If you get an error while downloading, with an error about HTTP, timeouts, connections, or the server, it's because the primary Open Fortress servers are down because of heavy traffic. You can switch to an alternative one by adding -u https://toast-eu-1.openfortress.fun/toast/ to the end of the upgrade command. Though, this can also be down.
If you've upgraded your game but you're getting an error about your game out of date when joining a server it can mean that either you're using a broken install, or the server is outdated. Both of these are possible. To fix a broken install, use the repair version of OFToast once and try again. If it didn't work it means the server is out of date.
If your game isn't launching, or there's missing textures, either run the repair version of OFToast once, or backup your configuration files, delete the .revision file in open_fortress, and re-run the upgrade command.
If your game isn't showing up in your Steam library, ensure Steam is fully restarted by clicking Steam -> Exit at the top left corner and opening it again. If that doesn't work ensure Source SDK Base 2013 Multiplayer and Team Fortress 2 is installed.
If you get a nil pointer dereference error, update murse. This error should only occur when the server is down on an outdated version.
If nothing happens, wait a minute. If the problem persists switch to an alternative server. You can switch to an alternative one by adding -u https://toast-eu-1.openfortress.fun/toast/ to the end of the upgrade command.

Manual Mirror Install

This involves a bit of elbow grease and should only be used as a last resort. The mirrors can quite often be out of date, and in the case of third-party ones can contain viruses.

Install Source SDK Base 2013 Multiplayer and Team Fortress 2. You can install the prior by going to your library, clicking the drop-down box labelled "Games" and ticking "Tools." Now search for "Source SDK" and it should show up. Ensure it is the correct version.
Navigate to your sourcemods folder. Your sourcemods folder is only on your primary drive! On Windows it’s typically in C:\Program Files (x86)\Steam\steamapps\sourcemods. On Linux it’s usually in ~/.steam/steamapps/sourcemods or ~/.local/share/Steam/steamapps/sourcemods. The console should start logging the files being added.
Download using the latest archive from here, or a trustworthy community member.
Extract the archive, such that gameinfo.txt is in sourcemods/open_fortress.
Fully restart Steam by clicking Steam -> Exit at the top left corner and opening it again.

Known Problems:

If the site you're downloading from requires bandwidth or you've run out of free downloads, you're SOL.
If your game isn't showing up in your Steam library, ensure Steam is fully restarted by clicking Steam -> Exit at the top left corner and opening it again. If that doesn't work ensure Source SDK Base 2013 Multiplayer and Team Fortress 2 is installed.
If you're getting an error about your game being outdated when joining a server, it's because either you're out of date or the server is. Try another server, or ensure you're running the latest version.