Arudha Lagna simply means how the world will see you as from your outer shell, or the image you will put out to the world. Arudha means 'Image', Lagna means 'rising sign or ascendant.' We, as individual souls have a separate prospection of ourselves in comparison to the world. If we think that we as a human being are spiritual and know a lot about life, then people outside like a stranger you come across might think of you as an ignorant bastard or someone who is a hippie or extremist in spiritual or religion realm. This image can also show you material gains as well, like people might look at you as a Donald Trump because of your big house and fancy cars you own. This way, while looking at the AL we can see that the person might have houses and cars because that's how people will look at him or her as, a rich person.
Most scriptures say AL is illusion or 'Maya', and people or astrologer think it is just an illusion of yourself, but that's not the case, 'Maya' simply means the material world that is away from the spiritual world. Since all of us or most of us want a fancy car, house, money etc.. Arudha Lagna can easily show us that. The ascendant of AL also shows the type of personality people think you are before talking to you or knowing you personally.
How do you find Arudha Lagna?
Well that is simple, you look at your ascendant lord in your birth chart, you locate it and see how many places it is away from the ascendant aka rising sign aka the 1st house. If for example your ascendant lord is Taurus and Venus is the 3rd house, since Venus is the ascendant lord by being the lord of Taurus sign, we would count three places from Venus. What do I mean by this? Venus is 3 places way from the ascendant as you count counter clock wise, using ascendant itself as the first place, then, you would count that many places from Venus, that means, the 5th house becomes your Arudha Lagna. You will simply put 5th house as the ascendant lord, but all the planets will be in their signs accordingly. It's like turning a lid to the jar, the planet will simply adjust to the ascendant.
If ascendant lord is in the ascendant, ie. Venus sitting in ascendant for Taurus ascendant then you will count 10 places away from Venus which simply becomes the 10th house. However, remember, Arudha Lagna can never be the ascendant of the main birth chart or the 7th house. If the ascendant lord is in the 7th house, then you will count 10 places from the 7th house which will simply become the 4th house as your AL which is the process of most astrologers;however after learning Jamini under Marc Boney I have come to adapt the theory of K.N. Rao about ascendnat lord being in 7th house then ascendant would become the AL, if ascendant lord is in ascendant then that is your AL. If ascendant lord is in the 4th house, then you counting 4 places from the 4th house will land you into the 7th house. I used to have certain belifs about AL and then found contarary informaiton which made more sense the normal pool of thought.