Having filed my first set of taxes after setting up a Solo-401k, I thought I'd share some things I learned (for the record, I filed my taxes through Turbotax, and, against the recommendation of various people, never consulted a CPA around tax law - so I basically DIYed it, and hope I didn't screw anything up!):
1) First of all, I accidentally contributed $85 too much as an employee contribution (turns out my old job contributed an unexpected extra payment to the 401k I had through them); I thought I'd hit $17500 on the nose (job 401K + solo 401k), but ended up at $17,585. (Lesson learned: don't max out your solo-401k until you are totally sure what any other 401ks contributions will be!) You have to do this by April 15.
I thought this would be a huge hassle to address, but Vanguard has a special form for "Mistake of fact" (Form s573, FYI). Note that this form is specifically for simple arithmetic errors; there is another form that Vanguard originally sent me that was for larger administrative errors, but that was the wrong one, and I had to do some snooping to get the s573). I sent that form in, and got a check for $85 a few weeks later. Didn't have to note this on my tax return, either, according to the instructions.
2) Something I discovered when preparing the tax return: for those planning on including your spouse as part of the plan (that is, contributing $17,500 x 2): it was NOT as simple as I expected. The IRS requires that for filing purposes (and only for filing purposes) you split your business into two - so two Schedule Cs, with separate income/expenses for your own and your spouses' portion of the business. Besides being an unexpected step because I never divided expenses between us and had to retroactively assign them based), this also became an issue because you or your spouse can't contribute more than the net income for each "business" (i.e., half-business) to your respective 401k. So, if your business nets $50,000, and your spouse accounts for $10,000 in net profit, he/she can't max out a 401k at $17,500. The max would be ~$10,000. So you have to be careful if you think you can just add on your spouse as a secretary or bookkeeper or something, and max out their 401k. You will have to account for how he or she "made" $17,500 on the business, and how you "made" $17,500.
In my case, my wife and I bring in about 50% of the revenue; however, I had never thought to split up expenses between us. Doing so is inherently imprecise and somewhat arbitrary since we share expenses; but I had to make sure that it evened out so that we both netted over $17,500.
3) The calculation of max Employer contribution could only be made after I'd done most of my taxes, because you need to factor in your expenses, employee contributions, and SE taxes. There is a worksheet to do this called the "Deduction Worksheet for Self-Employeed", in Chapter 5 of IRS Publication 560. Once I calculated this, I made my employer contribution in the same way I made my employee contribution, and that was that. Turbotax for small-business also calculates this, but I found it's presentation confusing so I completed the worksheet myself to double check the maximum allowable.
4) Finally, the solo-401k manager of a "One participant plan" (this includes individual and spouse) have to complete and send in IRS form 5500-EZ - I think the deadline is end of summer for the year before. However, Vanguard tells me (via their handy Guidelines for completing Form 5500, which they sent) that I only have to file this if my plan holds more than $250,000, which is not currently the case, and won't be for another few years at least.
I hope this is helpful to anyone, and if I'm wrong about anything, feel free to correct me!