Park tools website used to be my go to for all bike stuff. Won't have tribar assembly, but you'd be following the instructions specific to the bars you choose.

As for order of assembly, to be fair as you can't ride until it's all there then I'd tend to do it in one hit, but within reason you can go in any order.

I guess Chainset which then defines what BB you need , and then what B B in stall tools would be next if you had to. front/rear mechs at any time. Shifters need to happen after bars that happen after the frame/fork you've got. Add cables and chain and you're most of the way there. Don't forget things that are (essential) nice to have, like bottle cages, garmin mounts (the mount you may want to select after you figure out the bars and see what space you have. k-edge do a good range but I've bikes that nothing fits on, Black Zipties and the plastic basic gamin 'puck' just seem to work.

My list of frustrating holdups from past builds though:
Bar tape
End caps for mechanical cables
The plastic guide for cables under the BB
Frame lugs for external cables near top of downtube

Tools / etc to make sure you have:
Carbon assembly paste for seat and aero bar extensions.
Copper slip for use on all bolts/threaded things - will last for years - seriously, I do all own maintence, built 3 complete bikes and was 15 years ago I bought the tub and I've still a third left.
BB tool.
Large car mechanics torque wrench for BB
Small parktools torque wrench for all the other bolts (yea, I know it's debatable, but for me I am a huge advocate).
Adapter between 1/4 inch and 1/2 inch drive for the bits to be able to fit both above....
Nice quality set of allen keys (ok, i had some cheap ones that lasted 15 years but treat myself to some silca ones last year and they are nicer to use.
Electrical tape - just darn handy during assembly when you need 3+ hands to hold tools and parts.
Wire cutters - can't really get away with the cutting bit on pliers.
Chain tool - you'll need one to be able to adjust the chain you buy down to the length you need even if you are going to be using quick links going forward.
Bike stand. Be careful where you clamp to, and for BB install wait until you've got the wheels on and can stand on the floor to use the big tools to get the 35-50Nm generally needed.

Final tip.
When you are cabling the gears, make sure you add the metal ferule on the end that goes into the shifter, and for mechanical rear shift then cable it all with the barrel adjust in middle and make sure the shifter is set as it in the outer position, then push the rear mech gently in to about the 3rd cog, tighten the cable and let it go. You should find that the mech takes up the slack and is pretty much perfectly indexed, at least within the range of adjusting the barrel. If you don't do this then I find you need another hand to pull the cable with pliers whilst tightening the cable bolt. Similar with the front mech, but set shifter to the inner cog, hold the mech out slightly to almost (not fully) in line with the outer ring and clamp. This is easier before the chain goes on, and from here it's easier to make small adjustments later.

Hello all. So I just bought this GURU frame, fork and seat post for $300( def think I overpaid). It's used but in good condition.
Anyone know of a good resource to show steps/how to's on building tri bike. Searched some YT videos but didn't see any DIY type builds.

What are some of the next components I should be looking at buying that would be in the order of the install? Thanks a lot

If I were in your shoes, I'd try to find that actual frameset's full built bike manual from the vendor for the "shop build" OR find any other similar year tri bike build manual to follow that has similar style cable routing, bottom bracket, and such.

Next, obtain a work stand to build it on.

Next, obtain a strategy for shifting. Di2? 2x mechanical? 1x mechanical? Then get your parts lined up.
Same time, determine budget for front brake on normal or aero. Research the original shop build of that bike to see what the spec'd for the rear brake so you know you get one that will fit.

Next, budget/research your cockpit: base bar, brakes, extensions.

Start to follow the build procedure for whichever manual you got your hands on using all your parts. Youtube the individual procedures from the assembly manual.

Many of these manuals are photo heavy and step by step.

Bottom Bracket - the bearings that sit between the chainset and the frame.

Now here's where life is about to get really really miserable for you. There are almost limitless combinations here. Your frame constrains which BB standards (and there are way too many) you need (because the outer has to fit the diameter and width of the frame) and then the chainset runs through that, and there's plenty of differences there too.

On some chainsets the axle is so wide that it won't fit in some frames, so that's a no-no. Shimano chainsets have 24mm axles all the way through. SRAM chainsets have bigger diameters on the drive side to the non drive side. And they can have slight differences in the type of tool needed to fit them.

If your frame came with the BB fitted that could be a good start, but I'd personally put a new one in whilst the bike is stripped down unless it's known to be new and well fitted.

Google bottom bracket standards for a fair few good articles that run through the differences. And be grateful you didn't buy a cannondale.....

And the plus side is that if you've built the bike, then not only will maintenance be easy, but you'll actually notice as you're riding things before they fail, and know what it is that's failing so miss a lot less rides. Also when out on the road and something breaks, there's a lot better chance you can fix or botch a 'get me home' solution.

Thanks. Seems like I may have gotten myself into a headache...lol. Oh well, I'll get through it.

Ok, I think I need a little more info/insight into selecting my BB so it matches my crankset and my frame.
Where can I find details on these selections?

Thank you VERY much. What's BB?

So regarding the chainset/crankset are there any sizing variations where I could possibly choose the wring size for my bike?

Do you know what year the frame is?

Just looking at the picture, and a quick google then that looks like a ISO 68mm /BSA threaded frame. But you need to get measuring or find the spec sheet for the year that it was produced to confirm.

Which is fairly handy, as that gives you lots of options. So then choose a chainset - anything that's not BB30(ish - keeping it simple, i know there are exceptions) and then you can work out the specific BB you need.

Check out The complete guide to bottom bracket standards - BikeRadar and
Bottom Bracket Standards Chart (wheelsmfg.com)

For example, on my old Argon E-114 with BSA threaded frame (potentially what you have) I use a GXP BB and Quarq / SRAM dFour chainset. On the same frame I can pop in a shimano BB and run a shimano ultegra chainset (had to do this when the quarq was being fixed). So the frame will work with different axle shapes, but you need to swap the BB to match. Sorry, I'm now confusing more than helping, but this is the area I ended up doing the most head scratching when building as there's so many options

1) Confirm what your frame is.
2) Find a chanset you want.
3) check here or check on wheelsmfg for what BB you need.
4) Get a long winded response from me, 15 comments saying you shoudl be running 1x, three people telling you to go tubeless and your seat is too high and a two word response from jimatbeyond ;-)