My name is Dom and I've been playing League for half a year now. I'm writing this guide for those looking for something off meta but strong. I am relatively new to the game; however, I would count myself as a reliable source for Rek'sai as I have played quite a lot of her in the past few months. This is my first guide so feedback as to what was good and what needs to be improved would be great.

Why play Rek'sai top lane?
Rek'sai, if played right, is one of the strongest champions in the game IMO. She is easy to pick up, but difficult to master. Rek'sai's jungling is better than average, but I've noticed the top lane is her strong suit. On top Rek'sai can out damage and bully most top laners in the current meta. She also farms really well. I've found her kit to effective in most team comps and in winning the top lane. Her Q gives huge burst damage with a very low cooldown, and her E (with full fury) shreds tanks and is a solid finishing move. Her burrowed abilities are also useful good in the top lane. The Q is good poke early, while at the same time revealing enemies since you can't see without your minions. Your unburrow's knockup gives Rek'sai a nice jump on the opponent if he is trying to escape or engage. Her tunnels make chasing and fleeing Rek'sai difficult + landing skill shots. Rek'sai's passive allows her to easily outsustain most top laners, especially ones that are mana hungry. Finally her ult allows her to get in lane fast without using Teleport and keeping pressure on the map. The attack speed passive on it is also really good. Rek'sai's ult is on a relatively short cooldown so farming for 2 minutes and backing shouldn't punish you. It also allows Rek'sai to roam much easier after placing a few tunnels around the map.

What weakness should Rek'sai top laners be afraid of?
First off she is easy to gank pre-3. E is usually her third ability and burrowed it provides an escape. If a top laner gets Rek'sai low immediately (a good Rek'sai player will likely also make the other laner low) a strong early ganker (Lee sin, Jarvan IV, Rek'sai, Vi, Jax, or Zac) can engage her for a kill. However she can still keep up in gold with roams and CSing under tower, but an early death hurts.
Also she only has a knockup CC, and it requires burrowed form and has an 8 second cooldown on an enemy after hitting them (can be reused on other opponents not hit). This provides a window for the jungler or just the opposing top laner to burst her down without being stopped.
Lastly, Rek'sai's ultimate isn't a game changer and won't help win duels and team fights. The attack speed passive is useful, but a Renekton ult or a Darius dunk will cripple her.

-At level 1 trade with your opponent if they're melee. Play aggressive with your Q as it does a lot of damage all game.
-At level 2 burrow to heal and see if your opponent will get close enough for a knockup and Q autos. This will be your main combo against any opponent so get good at it.
-Try to hold onto your bite (E) until you have a full rage bar as it does true damage then. You can also use it as an execute even if it isn't fully charged.
-Before unburrowing for a knockup use your burrowed Q if it is off cooldown just for that bit of extra damage. That small amount of damage might be the difference between getting a kill or getting killed.
-Use your minions for vision while burrowed, landing consecutive Qs to poke may tilt a duel in your favor.
-While not farming try to stay in burrowed form as your tremor sense can detect a gank through the river if a ward isn't in place.
-Tremor sense can detect if your laner is moving through a bush
-You can tunnel from the river bush into tower range (Flash cannot). Learn other places your tunnel can extend really far.
-Rek'sai is great if played with Lulu and/or Yasuo. Rek can knockup multiple targets for Yasuo's ult, and Yasuo can knock them back up for the double knockup, and a Lulu can ult you for a potential 3rd knockup. By the end your knockup should have 2-4 seconds until usable again.
-Use Rek'sai's ultimate to get back into lane instead of Teleport. This will force an enemy TP if they have it, giving you more global pressure.
-Your team can see your tremor sense, this can give you an edge on your opponents if they're trying to ambush or if you're setting up an ambush
-Setup tunnels for escapes so you can use the tunnel while off CD for engages
-Aggressive early game usually goes in your favor

Let me know feedback and how your game went. Leave any questions for me and I will get back ASAP!

*I am gonna try to record a gameplay and upload it to youtube as the Rek'sai Top games I have seen aren't the best examples of what he can be*