This guide is designed for playing as Nasus, laning by yourself while one of your teammates jungles. Nasus is an exceptional solo-er due to three main parts of him:
1) His passive gives him lifesteal starting from level one, this means that health is not one of the main issues.
2) Wither is one of the best crowd controls in the entire game! Slowing the enemy while reducing attack speed is one of the best moves anyone can use in a gank.
3) His ultimate, Fury of the Sands grants him bonus health, lifesteal, and attack damage. This can be used either while ganking, in team fights, or running away from a gank.
*NOTE*
This guide is currently unfinished and was published prematurely by accident.

Multiple combinations of Summoner Spells can be used for multiple situations that you maybe encounter while soloing.
Flash can be used because of its ability to let you chase the enemy during ganks or to escape from being ganked yourself.

Teleport has the use of letting yourself go back for a quick heal/item shop and going back to defend your lane. This is its main use, but also it can be used to teleport to cover turrets for teammates who died or recalled. Another use of it, which is rarely used is teleporting to friendly sight wards in order to gank in different lanes. I recommend to always pick teleport with a Summoner Spell of your choice (based upon which characters are on your team, which character is your teams jungler, and your skill level).

Exhaust has the uses of slowing enemies during ganks, and slowing enemies that are ganking you. Other than that, there are not many other uses that I know of.

Although I am not a fan of Clarity, it has its uses in the beginning if you wish to be an offensive Nasus and use Spirit Fire often for farming and for controlling the enemies. It is a worthless Summoner Spell other than that use and is rarely any use late game because by that time you won't have to worry about your mana.

Ignite can be used during ganks to finish off players with low health whom are getting out of Nasus' reach, and can also be used as a harass to get enemy players to recall, leaving the lane to be a one versus one, which if became a fight, you will win because of Fury of the Sand's effects: lifesteal, extra health, and bonus attack.

Revive is a useful summoner spell from early to late game. Although it should not be used early game (when the respawn time is a couple seconds), it does have its advantages late game and can possibly save your team if the enemy scores an ace. Another possibility is the "avenge" combination, which includes a teleport, so then your are able to respawn, teleport, and finish the enemy who has killed you.

Clairvoyance shouldn't be used if you are soloing because there are other, more useful spells to choose from. Unless you're going a support Nasus (which I have never heard of) then it is best to avoid selecting this Summoner Spell. The few uses it has whilst soloing is to check for possible ganks, and to make sure the area is clear for your jungler to gank.

Ghost is one of the more useful spells that you can chose from for this situation. It has three main uses, which are chasing an opponent during a gank, running away from a gank, and going back to your lane after a death or a recall.

Promote is one of the Summoner Spells I recommend for soloing as Nasus. It is almost essential for one to choose this. It can be used for promoting a minion to push up in your lane, defending your turret (opposing minions will not attack the turret until all of your minions are killed). Additionally, it can replace one of the uses of teleport, giving you enough time to recall and get back to your tower. It takes a short, but extended period of time (about 20 seconds) to kill this promoted minion, so you will have enough time to get back without any damage to your turret.

Smite is of no use, other than getting easy gold from a minion kill every seventy seconds. It could be used later game against super minions, but other than that, it is a useless spell for the soloing position and just is a waste of a Summoner Spell slot if one were to choose it.

Cleanse is a good spell to pick, and can be used to "cleanse" yourself if you are under crowd control during a gank. It can work with flash fairly well if you choose to become offensive.
Heal can be a useful spell while soloing, but with Nasus' passive, he will be more mana hungry than have a need for health. It can save you if you're being ganked or losing in a one versus one, but other than that, its uses are limited.

I do not recommend Surge, plainly because it is a offensive spell, and even thought can give you a maximum of plus seventy-eight ability power with a thirty-five percent higher attack speed, it is only useful during fights, which is what solo-ers want to avoid other than ganks from the jungler. It can be useful, but you should not need a spell to power you up if you have Fury of the Sands and are a higher level than the two players against you.

This skill sequence was chosen completely based upon defense and holding your lane well whilst still being able to farm. Siphoning Strike is Nasus' main farming tool and should be maxed out first, while also occasionally getting Spirit Fire to clean up attacks on turrets. Also based upon my experiences of playing, the opposing players will tend to be scared of Spirit Fire, and move out of its range. Therefore, ranking it up occasionally is useful for cleaning up minions attacking your turret, and it also has the use of moving enemy champions out of a certain area, if they have the habit of doing so (as stated above). Defending until your jungler ganks is keen for your survival while soloing a lane.

Farming while soloing should be simple, but as playing as Nasus, one wants to save their mana and not use Spirit Fire for farming, but rather for defense. When you are a higher level (10+) the use of Spirit Fire for farming may work out without wasting all of your mana, but before that it is important that the player only uses Siphoning Strike to farm, as it doesn't waste large amounts of mana.

After defending until your jungler is available for a gank, depending if you are in the top or bottom lanes, have the jungler avoid going down the river due to sight wards if possible, in order to ensure the best gank possible. Also, you should have the other team pushing up, maybe even at your tower. You first use a combo of Spirit Fire and Siphoning Strike to clean up the minions. You then use Fury of the Sands and Wither to target one of the enemy champions (most likely the squishier of the two) and try to kill them. Depending whether or not you succeed, your jungler should use crowd control on either the injured champion or the remaining one (if you had slain the squishier one) and then try finish them. If played right, you will use up their flashes (if they have any) and/or kill and injure both of them.
An example with the classic jungler Udyr is he enters from the jungle exit closest to your tower while you attack the squishier champion. Udyr will then use his Bear Stance to rush into the fight and stun the champion you injured so you both can finish them off. After that, depending on the Summoner Spells you chose, you can either use another Wither (if it's not on cooldown), use an Exhaust (if you chose it as a Summoner Spell), or have Udyr use his Bear Stance again, switching to Tiger Stance when he reaches the enemy to ensure maximum damage dealt.
*Note*
You should know the character that is jungling as well as you know Nasus, an easy way to do so is to look for your jungler in Hanaho's guide. (Here it is.)

I chose mainly defensive masteries since a majority of the time Nasus will be defending his turret. If one chooses so, you can possibly make more of an offensive mastery in order to make ganks as deadly as possible. If chosen, then it would be best to go 21-9-0, still keeping some defensive masteries for defending yourself.