How do I declare my income?   Unless you incorporate or are in a partnership the income and expenses go on a Sch C as part of the form 1040.

Can I do a total sum or do I have to detail each invoice?  All you need to enter in an income tax program is the total income ... keeping track of the individual invoices is done in a bookkeeping program. 

If you are new to being self employed, are not incorporated or in a partnership  and  are acting as your own bookkeeper and tax preparer you need to get educated ....  

If you have net self employment income of $400 or more you have to file a schedule C in your personal 1040 return for self employment business income. You may get a 1099-NEC for some of your income but you need to report all your income.  So you need to keep your own good records. Here is some reading material……

IRS information on Self Employment….
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center 

Publication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf 

Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf 

Home Office Expenses … Business Use of the Home

https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction

https://www.irs.gov/pub/irs-pdf/p587.pdf

Publication 463 Travel, Gift, and Car Expenses

https://www.irs.gov/pub/irs-pdf/p463.pdf 

Publication 946 … Depreciation

https://www.irs.gov/pub/irs-pdf/p946.pdf