This is general template for creating your own custom erc20 token.

CustomERC20.sol

// SPDX-License-Identifier: MIT
pragma solidity >=0.5.0 <0.8.0;


interface ERC20{
    function balanceOf(address _owner) external view returns (uint256 balance);
    function transfer(address _to, uint256 _value)  external returns (bool success);
    function transferFrom(address _from, address _to, uint256 _value) external returns (bool success);
    function approve(address _spender  , uint256 _value) external returns (bool success);
    function allowance(address _owner, address _spender) external view returns (uint256 remaining);

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

contract YourCustomToken is ERC20{

    uint256 public totalSupply;          //Total coins in circulation
    string public name;                   //fancy name: eg Dev's Bucks
    uint8 public decimals;               //How many decimals to show.
    string public symbol;                // Symbol : DBK (for Dev's Bucks)
    mapping (address => uint256) public balances;    
    mapping (address => mapping (address => uint256)) public allowed;

    constructor(uint256 _initialAmount, string memory _tokenName, uint8 _decimalUnits, string  memory _tokenSymbol) {
        balances[msg.sender] = _initialAmount;               // Give the creator all initial tokens
        totalSupply = _initialAmount;                        // Update total supply
        name = _tokenName;                                   // Set the name for display purposes
        decimals = _decimalUnits;                            // Amount of decimals for display purposes
        symbol = _tokenSymbol;                               // Set the symbol for display purposes
    }



    function balanceOf(address _owner) public override view returns (uint256 balance) {
        return balances[_owner];
    }

    function transfer(address _to, uint256 _value) public override returns (bool success) {
        require(balances[msg.sender] >= _value, "token balance is lower than the value requested");
        balances[msg.sender] =  balances[msg.sender] -  _value;
        balances[_to] =  balances[_to] +  _value;
        emit Transfer(msg.sender, _to, _value); 
        return true;
    }

    function approve(address _spender, uint256 _value) public override returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value); //solhint-disable-line indent, no-unused-vars
        return true;
    }

    function allowance(address _owner, address _spender) public override view returns (uint256 remaining) {
        return allowed[_owner][_spender];
    }

    function transferFrom(address _from, address _to, uint256 _value) public override returns (bool success) {
        uint256 allowance = allowed[_from][msg.sender];
        require(balances[_from] >= _value && allowance >= _value, "token balance or allowance is lower than amount requested");
        balances[_to] = balances[_to] + _value;
        balances[_from] = balances[_to] - _value;
        allowed[_from][msg.sender] = allowed[_from][msg.sender] - _value;
        emit Transfer(_from, _to, _value); //solhint-disable-line indent, no-unused-vars
        return true;
    }
}