Clone $PROJECT_DIR/assets/Steam/achievements/template.svg.
Edit the text. If you use non-ascii characters (e.g., emojis), you should convert them to path data: Path -> Object to Path.
Object -> Align and Distribute ...
- Relative to: Choose "Page".
- Press the icon having the tooltip: "Center on vertical axis".
- Press the icon having the tooltip: "Center on horizontal axis".

Note:

You should NOT use the option "Save and restore window geometry for each document" (Edit -> Preferences -> Interface -> Windows).
The previous steps are not mandatory, but you should follow them. Doing them helps you catch potential visual issues.

Tips for checking new svg files: Check "sodipodi:docname" in the new svg file. If it does not match the filename, it's very likely the svg file was cloned from another file, but it was not realigned and double-checked in Inkscape (Inkscape automatically updates "sodipodi:docname" when saving, so if you open the new file, realign the position, and save it, "sodipodi:docname" will match the filename).

Add achievements

These steps are mandatory:

Add the new icon (.svg file) to $PROJECT_DIR/assets/Steam/achievements/icons.
Add an entry to $PROJECT_DIR/src/Achievements/AchievementData.json.
- It should match the information of the Steam achievement, if applicable.
- Order the new achievement entry thematically.
Add an entry to $PROJECT_DIR/src/Achievements/Achievements.ts.
- Match the order of achievements in AchievementData.json.
- Icon must be the name of the .svg file.
- NotInSteam must be true.
Run pack-for-web.sh in $PROJECT_DIR/assets/Steam/achievements.
When committing, remember to commit the changes in $PROJECT_DIR/dist/icons/achievements.

Note: If you add a new SFx.1 achievement, you must add its ID to SFAchievementIds in $PROJECT_DIR/src/Achievements/Types.ts.