Slither.io is free to download and play. Players can enjoy core gameplay without any charges. However, the game does include optional in-app purchases, primarily skins and custom designs for your snake, which allow for personalization and enhanced visual appeal. These purchases are entirely optional and don't affect the gameplay itself.

Additionally, slither.io might show advertisements during gameplay, which help support its free model. These ads are typically seen between gaming sessions and are not intrusive during the actual play time. Players can opt for a premium version to remove ads if available. It's designed to maintain an enjoyable experience whether or not you choose to spend money on additional content or remove ads.

Installation on iOS Devices: To install slither.io on iOS devices, ensure your device is running iOS 10.0 or later. Simply open the App Store on your device and search for "slither.io" using the search bar. Once the app appears, tap on the icon and select "Get" to start the download. You may need to confirm using your Apple ID password or Touch/Face ID. After installation, the game will request permissions for sending notifications; ensure to allow these if you want alerts. By default, slither.io does not request access to storage or your personal data.

Installation on Android Devices: For Android users, slither.io is compatible with Android 4.4 and above. Open the Google Play Store and type "slither.io" in the search bar. Locate the app in the search results, and press "Install." Wait for the download and installation process to finalize. Upon opening the app for the first time, it might ask for network permissions to interact with online servers; this is necessary for multiplayer gameplay. Follow prompted steps to grant any required permissions, if any, and your game will be ready to play.

Step 1: Upon launching slither.io, you'll be greeted with the main menu where you can customize your snake. Choose from various skins or design your own by clicking on "Change Skin."

Step 2: Enter a nickname for your snake, which will be visible to other players during the game. You can type this directly in the designated field on the main screen.

Step 3: To start playing, select "Play" from the menu. Your snake will appear on the map, where you'll begin to navigate around other snakes, collecting glowing orbs.

Step 4: Control your snake's movements by simply dragging your finger across the screen (on touch devices) or using the mouse (on computers) to guide its direction. Eating more orbs increases your snake's size.

Step 5: Avoid crashing into other snakes. If your snake's head touches another snake, your game is over, and your final length is recorded. Use quick maneuvers to encircle or trick other snakes, allowing you to gather their orbs if they crash into you.

Tip 1: Focus on collecting the small orbs scattered around the arena, especially early in the game when your snake is smaller and more agile. These orbs help you grow while avoiding conflict with larger snakes.

Tip 2: Use the boost feature strategically by double-tapping or clicking to move quickly. While boosting consumes some length, it's a vital tactic to escape danger or aggressively circle another snake to force them into you.

Tip 3: Adapt to your growing size by changing tactics; as your snake grows longer, prioritize strategic positioning over speed, using your body to create barriers and force smaller snakes into unfavorable collisions.