Many people who start to learn electronics become confused. First we teach them that electrical current is composed of a flow of electrons. We go on to teach that electrons are negatively charged and they move towards the positive charge. This may seem to them to be a little backwards but then we show them the symbol for a solid state diode and the arrow points towards the negative charge to be forward biased. So the electrons are flowing from negative to positive but the arrow is pointing from positive to negative.
The blame for this is usually placed on Benjamin Franklin who was the first person to start marking the charges as positive and negative in about 1750. Franklin had no way of measuring which way the current flowed so he had a 50/50 chance of being right but most people thinks he missed.
If the electron flow is the same as current flow and the diode symbol is an arrow then it would be pointing in the wrong direction. But if current flow and charge flow are the same thing then the negative charge is going towards the positive charge and the positive charge is going towards the negative charge.
Frequently current flow is compared to water flow with the source of the water being the negative side; the water is compared to electrons, and the drain as the positive terminal.The illustration is a good one but like any illustration it is not perfect.There is not a pile of electrons that start flowing down the conductor like water in a pipe. The excessive negative charge knocks an electron out of an atom. This leaves a positive charge to that atom so an electron enters to replace the missing electron. The electron that was knocked out will migrate towards the positive side and it will knock out another electron and replace it so current flow is a series of electrons being knocked out of orbit and being replaced by another. This action will cause the negative charge to move towards the positive terminal and a positive charge moves towards the negative terminal.
Try to visualize a string of atoms with the electrons (negatively charged) going around the nuclease (positive charged protons and neutrons). Now the positive charged terminal grabs an electron out of the atom closes to it making that atom positively charged. Meanwhile the negative charged terminal inserts an electron out of the atom closes to it making it negatively charged. The negatively charged atom can’t hold all those electrons so it pops one out becoming neutral again and the free electron is being pushed by the negative charge and pulled by the positive charge so it moves in the direction of the positive charge and joins in orbit in another atom knocking an electron out of it. The positively charged atom next to the positively charged terminal wants an electron and grabs it from the atom next to it thus making that atom positively charged and it becomes neutral again.
The process continues with the negative charge moving towards the positive terminal and the positive charge moving towards the negative terminal. So there is a negative flow and a positive flow but in the opposite direction.
This is a little over simplification on how it works but basically that is how electricity moves in a conductor.
So if you consider the symbol of a diode to be an arrow pointing the direction of current flow then just remember it is pointing in the direction of the positive charge flow known as Conventional Current Flow.
Now if this still sounds confusing to you then look at the symbol as a horn and not an arrow with electrons flowing out of it. This might help you remember which way to place the diode.