What you are installing
An expert advisor (EA) is an automated trading program; a custom indicator is an add-on that draws extra analysis on your chart. Both come as files you place into MetaTrader's folders. Installing them is straightforward, but where the file comes from matters enormously.
Before anything else, an honest warning: the world of downloadable EAs and indicators is full of scams and outright malware. Treat every file with suspicion until proven otherwise.
The numbered steps
Step 1: In MetaTrader, open File → Open Data Folder. This opens the exact folder the platform reads from — do not guess the path elsewhere.
Step 2: Open the "MQL4" folder (MT4) or "MQL5" folder (MT5). Inside, put an EA file into the "Experts" subfolder, or an indicator file into the "Indicators" subfolder.
Step 3: Copy the .ex4/.ex5 (compiled) or .mq4/.mq5 (source) file into the correct subfolder.
Step 4: Back in MetaTrader, right-click the "Navigator" panel and choose "Refresh". Your EA or indicator now appears in the Navigator list, ready to drag onto a chart.
Getting it onto a chart
Drag the EA or indicator from the Navigator onto an open chart, adjust its settings in the window that appears, and click OK. For an EA to trade, you also need auto-trading enabled — see the separate guide on that.
If the item does not show up after refreshing, double-check you put it in the right subfolder and that the file matches your platform (MQL4 files do not run on MT5 and vice versa).
An honest risk note
A downloaded EA can place real trades with your real money the moment you enable it, and a malicious file can steal credentials or drain an account. Never install anything from an untrusted source, never enter your account password into a third-party tool, and never run an unknown EA on a live account. Test everything on a demo first, and be sceptical of any EA sold with promises of guaranteed profit — those promises are the surest sign of a scam.