Hi,

Please refer to the code below:

Python

from odoo import models, fields

class CrmLead(models.Model):
    _inherit = 'crm.lead'

    x_custom_field = fields.Char(string="Custom Field")
    # Add Your Custom fields

XML:

<record id="view_crm_lead_tree_custom" model="ir.ui.view">
    <field name="name">crm.lead.tree.inherit.custom</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_leads_view_tree"/>
    <field name="arch" type="xml">
        <xpath expr="//field[@name='name']" position="after">
            <field name="x_custom_field"/>
        </xpath>
    </field>
</record>

Hope it helps.

To show your custom fields in the crm.lead list view, you need to inherit the tree view and add your fields.
<record id="crm_lead_tree_view_custom" model="ir.ui.view">

    <field name="name">crm.lead.tree.view.custom</field>

    <field name="model">crm.lead</field>

    <field name="inherit_id" ref="crm.crm_lead_view_tree"/>

    <field name="arch" type="xml">

        <xpath expr="//tree" position="inside">

            <field name="x_custom_field"/>

        </xpath>

    </field>

</record>

Make sure your model extension correctly adds the new fields:
from odoo import models, fields

class CrmLead(models.Model):

    _inherit = 'crm.lead'

    x_custom_field = fields.Integer(string="Another Field")